The Energy Conversions in a Hybrid-Propulsion Vehicle with Planetary Gear Set

 2
Barcelona, Florence, Lisbon, Oxford and Stockholm. In Rome, a fleet of 40 electric buses - 
the biggest in Europe - and various hybrid buses are in service in the centre of the town. 
The Japan Automobile Makers Association (JAMA) has measured a huge - though still 
not overwhelming - increase in environmental preferences among vehicle buyers, and fuel 
(energy) efficiency now tops the preference list there. 
With the increase of the number of vehicles in circulation, the necessity of alternatives to 
conventional vehicles propelled by internal combustion engines (ICEs) is more and more 
obvious. Great part of the oil reservoirs find in a politically unstable area like the Middle 
East, and however they cannot always last in order. The oil price shock experienced by the 
global marketplace in the mid-1970's had however a profound effect on automobile design, 
toward models of increasing fuel efficiency. This experience was reinforced by the adoption 
e.g. in the United States of Federal Average Fuel Economy Standards, which provided 
requirements for automobile manufacturer sales fleet fuel economy.  
The dangers for the health constituted from nitrogen oxides and other substances emitted 
from the vehicles are known, and are increasing the worries for the "greenhouse effect", 
promoting global warming and caused by the carbon dioxide emissions. Federal and state 
clean-air regulations also required automakers to respond with automobiles that emitted 
lower levels of tailpipe hydrocarbons, carbon monoxide, and oxides of nitrogen. 
One of the main driving forces which has stimulated the research and development of the 
electric vehicle has been the California Air Resources Board (CARB) mandate, which 
dictated that 10% of vehicles sold in California would be zero emission vehicles (ZEV) by 
the year 2003. The CARB mandates have also been adopted by the states of New York, 
Massachusetts and Vermont. Texas is considering whether to adopt the mandate.  
In Japan the Ministry of International Trade and Industry (MITI) in 1997 had set the goal 
of introducing 200,000 electric vehicles (EVs) by year 2000. Indications were that about 
5.7% of the Japanese domestic car products for 2004 should be EV. In Europe local 
authorities are heavily involved in alternative fuelled vehicle introduction by adopting EVs 
for service applications such as postal delivery services and city centre transportation 
systems. Total European EV sales are anticipated to be around 700,000 units by year 2004. 
Nevertheless, no more than a derisory 10,855 of Europe's 150 million registered motor 
vehicles (1997 figures) are electric powered. Most are found in France (3,500), Switzerland 
(2,500) and Germany (2,200). A large part of these operate within the "captive" fleets of 
local authorities or private companies. The electric car fleet of a company such as Electricité 
de France alone consists of 1,000 EVs, or 30% of all electric-powered vehicles in France. 
The use of EVs by private car drivers remains at an embryonic stage and the market is 
showing no signs of picking up. PSA-Peugeot-Citroen, Europe's leading manufacturer of 
"mass produced" EVs, recorded sales of just 1,400 electric cars in 1996. A figure which it 
was unable to match in 1997. In Europe, concrete measures proposed to promote electric-
powered vehicles include support for battery hire schemes (proportionally the most 
expensive component of the vehicle), the awarding of purchasing premiums, reduced VAT 
and other taxes, lower insurance premiums (given the lower speed), and the imposition of 
EV quotas in public sector fleets, etc. 
Electric vehicles also pose a significant opportunity for power semiconductors producers. 
But this is a long-term hope, as electric vehicles suffer from relatively poor batteries 
allowing a range of only some hundred kilometres, and prices which today are not 
competitive on the market. The main propulsion (the motors that drive the wheels) will 
unduobtedly be an electronic drive which will most likely contain custom-made IGBTs
1
 
mounted on an isolated substrate. Power stages will offer a compact, lightweight, rugged and 
                                                           
1
  Insulated Gate Bipolar Transistor 
 3
efficient "plug in" solution that can be eventually hub-mounted for two or four wheel drive. 
Additionally, the required battery chargers and power distribution infrastructure will offer 
tremendous sales opportunities for fast, low-loss switches. 
The auto makers complied with the CARB mandate - but results have been predictable. 
Despite the best efforts of auto makers to produce a viable EV, the long-standing problem of 
battery range remained insurmountable. In U.S.A. Honda stopped making its EV-Plus after 
just two years, and in January 2000 GM announced it would be ceasing production of the 
EV-1 in the course of the year due to a lack of demand. More practical technologies - 
hybrids and fuel cells - have been developed in the mean time, not to mention stringent 
efforts on the part of auto makers to produce far cleaner cars powered by conventional 
powerplants. 
CARB now has to decide whether to pursue or to back away from the second stage of the 
CARB mandate. The Board has to decide by autumn 2000 whether to keep, modify or drop a 
rule requiring that up to 10% of new cars and light trucks sold in the state are zero-emissions 
vehicles. The rule would affect all auto makers selling at least 4,501 new cars a year in the 
state, and would take effect with 2003 models. Moreover the CARB's decision has 
implications for at least three other states which have matched California's requirement.  
The State of California has scaled back its zero-emission requirements twice since they 
were adopted in 1990. Originally, the regulations required that ZEVs make up 2% of a 
manufacturer's fleet of new cars starting in 1998, 5% in 2001 and 10% in 2003. In 1996, 
after a campaign by auto and oil companies, CARB dropped the 1998 and 2001 
requirements. In 1998 it voted to let manufacturers meet part of the 2003 requirement - up to 
6% - by selling "super ultra low-emission vehicles" (SULEV). 
To fulfil the CARB mandate with EVs, auto makers would have to produce more than 
30,000 electric cars or light trucks to meet the requirement in 2003, about 10 times the 
number sold or leased in California over the past three years [Bursa, 2000]. Indeed, few EVs  
have been sold as replacement for gasoline-powered cars. Most of the ones supplied to 
private consumers  have been used as a family's second car, for commuting, errands and 
other short trips, with the conventional vehicle retained for long journeys. For the time 
being, the best chance auto makers have of finding customers for EVs is direct to public 
utilities, such as power companies or other government departments. 
The solution offered by electric vehicles
2
 thus is useful only in few cases: their limited 
range makes their introduction difficult, the electrochemical batteries need to be recharged 
with low currents, and this increases the recharging time; on the other hand, the substitution 
of battery groups while in service is expensive. These limitations will probably delay the 
dissemination of electrical vehicles previewed by the timetable of some mandates. 
Progress has been made in designing cleaner vehicles which are safer and more energy-
efficient. Engine manufacturers have begun work on designing and controlling combustion 
                                                           
2
 Through the early period of the automotive industry until about 1920, electric automobiles were competitive 
with petroleum-fueled cars particularly as luxury cars for urban use and as trucks for deliveries at closely 
related points, for which the relatively low speed and limited range, until battery recharge, were not 
detrimental. Electrics, many of which were steered with a tiller rather than a wheel, were especially popular 
for their quietness and low maintenance costs. Ironically, the death knell of the electric car was first tolled by 
the Kettering electrical self-starter, first used in 1912 Cadillacs and then increasingly in other gasoline-engine 
cars. Mass production, led by Henry Ford, also reduced the cost of the nonelectrics. Electric trucks and buses 
survived into the 1920s, later than passenger cars, especially in Europe. Electric automobile prototypes 
reappeared in the 1960s when major U.S. manufacturers, faced with ultimate exhaustion of petroleum-based 
fuels and with immediate rising fuel costs from the domination of Arab petroleum producers, once again 
began to develop electrics. Both speed and range were increased, and newly developed fuel cells offered an 
alternative to batteries; but by the mid-1980s electric automobiles had not become a part of the automotive 
industry's output. Most industrial in-plant carrying and lifting vehicles, however, were electrically powered. 
 4
so as to reduce emissions of CO
2
 and other greenhouse gases, but despite efforts to produce 
cleaner cars, the pollution caused by transport has now reached an alarming level and in 
Europe some EU Member States are concerned and are envisaging more restrictive 
legislation, like that which applies in California. 
The Kyoto Protocol, adopted by 174 countries in December 1997, has set the first precise 
targets in terms of levels and dates. Between 2008 and 2012, Europe is to cut greenhouse gas 
emissions by 8%, the United States by 7% and Japan by 6%. Despite this progress, the 
Kyoto results fall well short of what was proposed by the Europeans who were prepared - 
provided the other industrialised nations followed suit - to cut emissions by 15%. In July 
1998 the European Automobile Manufacturers Association (ACEA) finalised a voluntary  
 
commitment with the European Commission to achieve 140g CO
2
/km on average for new 
cars by 2008 (see diagrams above). 
The European Commission itself supports research activities in the clean-and-more-
efficient-car field. An example of the results obtained with these efforts is the demonstration 
vehicle Renault FEVER (Fig. 1) - one of the first experimental electric vehicles powered by a 
fuel cell - public presented on 29 September 1998 and realised by the French manufacturer 
in collaboration with the Italian companies De Nora (responsible for fuel cell production) 
and Ansaldo (assembly of secondary systems and hydrogen tank with the fuel cell), Air 
Liquide of France (manufacture of the hydrogen tank), Volvo of Sweden (simulations) and 
the Paris School of Mines (definition of the system's 
operating parameters). Renault, De Nora and Air 
Liquide are also cooperating with other partners on 
the EU's HYDRO-GEN project: coordinated by the 
French car manufacturer PSA, this aims to develop 
another type of vehicle using a new generation of 
fuel cells and compressed hydrogen. In addition to 
HYDRO-GEN, European programmes are also 
supporting projects such as FCBUS (the fuel cell 
bus), coordinated by Air Liquide, and CAPRI, an 
initiative coordinated by Volkswagen which is based 
on a new method of hydrogen supply [RTD info 21, 1999]. 
The motor industry is a key sector of the European economy. The car industry itself 
employs 1.6 million people while the employment in the automotive sector represents 8.2% 
of the total employment of the manufacturing industry and the value added of the automotive 
sector represents 9.3% of the value added of the manufacturing sector. 
The contribution of the automotive industry to the EU GDP is 1.61%.  
Fig. 1  Renault FEVER 
 5
 
 
* Updated in June 1999 - Provisional figures  
Source: ACEA Member Companies' Annual Reports and 
ACEA Estimates 
 
 
 
 
Fig. 2 Key Figures: ACEA (Association des 
Constructeurs Européens d' Automobiles: 
European Automobile Manufacturers 
Association) members in the World 
 
Investments represent around 8% of the turnover and R&D around 5% of the turnover. In 
average, there is a value added per employee of 55,000 Euro. 
 
Table 1  Automobile Production in the European Union (in units) 
 1995 1996 ch% 
96/95 
1997 ch% 
97/96 
1998 ch% 
98/97 
1999 ch% 
99/98 
Passenger 
Cars 
 
12,636,067
 
13,061,348 
 
3.4% 
 
13,451,272
 
3.0% 
 
14,510,472
 
7.9% 
 
14,933,470 
 
2.9 % 
Light 
Comm. 
Vehicles 
 
 
1,318,462 
 
 
1,393,245 
 
 
5.7% 
 
 
1,570,265 
 
 
12.7% 
 
 
1,675,315 
 
 
6.7% 
 
 
1,616,931 
 
 
- 3.5% 
Trucks 348,577 310,204 -11.0% 334,562 7.9% 379,094 13.3% 394,211 4.0 % 
Buses 30,519 32,001 4.9% 36,672 14.6% 35,397 -3.5% 33,788 - 4.5% 
TOTAL 14,333,625 14,796,798 3.2% 15,392,771 4.0% 16,600,278 7.8% 16,978,400 2.3 % 
Source : National Associations/ACEA (Association des Constructeurs Européens d' Automobiles: European Automobile 
Manufacturers Association), updated on 16 June 2000. 
 
 
Table 2  EU Automotive Industry: Structural Data 
Total Activities in EU and Controlled from EU (1997) 
 Manufacture of
Motor Vehicles 
Manufacture of 
Bodies & Equipm. 
Total  
Turnover 321 112 433 Billion of ECU
Production Value 276 105 381 Billion of ECU
Value Added at factor costs 70 36 106 Billion of ECU
Labor Costs 45 26 71 Billion of ECU
Gross Wages & Salaries 35 20 55 Billion of ECU
Total Employment 1,121,270 764,110 1,885,380  
 
 
 
 6
1.3 The U.S. Partnership for a New Generation of Vehicles 
 
In September 1993, along with attempting to reduce the environmental impact of 
passenger vehicles and to improve the global competitiveness of American industries, 
President Clinton and the three major American motor manufacturers - Chrysler 
Corporation, Ford Motor Company, and General Motors -, along with several government 
agencies, including the national labs, launched the "Partnership for a New Generation of 
Vehicles" (PNGV) and in 1995 the Federal Government earmarked US$300 million for 
research directly linked to this initiative. All the major Japanese car companies are also 
investing heavily in this area. 
One of the three main goals of PNGV is the development of a mid-size five passenger 
sedan with the same safety, performance and conveniences of a conventional vehicle, but 
with three times the current fuel economy - a goal of nearly 80 miles per gallon (34 
kilometers per liter). The three specific, but interrelated goals of PNGV are: 
 
Goal 1: Significantly improve national competitiveness in manufacturing: improve the 
productivity of the U.S. manufacturing base by significantly upgrading U.S. manufacturing 
technology, including the adoption of agile and flexible manufacturing and the reduction of 
cost and lead times, while reducing the environmental impact and/or improving product 
quality. 
 
Goal 2: Implement commercially viable innovations from ongoing research in conventional 
vehicles: pursue technology advances that can lead to improvements in the fuel efficiency 
and reductions in the emissions of standard vehicle designs, while pursuing advances to 
maintain safety performance. Research will focus on technologies that reduce the demand 
for energy from the engine and drivetrain. Throughout the research program, the industry has 
pledged to apply those commercially viable technologies resulting from this research that 
would be expected to significantly increase vehicle fuel efficiency and improve emissions. 
 
Goal 3: Develop vehicles that can achieve up to three times the fuel efficiency of 
comparable 1994 family sedans: increase vehicle fuel efficiency to up to three times that of 
the average 1994 Concorde/Taurus/Lumina automobiles with equivalent cost of ownership 
adjusted for economics. 
 
The Partnership's target was to develop a concept vehicle by 2000 and a production 
prototype by 2004. This ten-year time frame for the PNGV would represent an extremely 
rapid development effort to produce a revolutionary change in automotive transportation. 
 
The parameters for the New Generation of Vehicles with up to Triple Fuel Efficiency are: 
 
Vehicle Attribute      Parameters 
 
Acceleration      0 to 60 miles per hour in 12 seconds 
Number of Passengers    up to 6 
Operating Life     100,000 miles (minimum) 
Range      380 miles on 1994 Federal Drive Cycle 
Emissions      Meet or exceed EPA Tier II  
Luggage Capacity     16.8 cubic feet, 200 pounds 
Recyclability     80 percent 
Safety      Meet safety requirements* 
 7
Utility, Comfort, Ride & Handling   Equivalent to current vehicles 
Purchase & Operating Cost    Equivalent when adjusted for economics 
Curb Weight       up to 40% weight reduction over baseline 
(3200 pounds) 
Aerodynamics      0.20 Cd (drag coefficient) (Innovative 
styling, shielding) 
Friction (Rolling Resistance)    0.005 (Improved tires and friction 
management) 
Fuel Efficiency     up to 80 mpg (Metro-Highway) 
 
 
*Federal Motor Vehicle Safety Standards (FMVSS) 
 
 
The Government PNGV Secretariat has published on 29 November 1995 the "PNGV 
Program Plan" which details the goals, planning, milestones, and resources associated with 
the PNGV program. Regarding the Goal 3 of the Partnership, the research undertaken in 
pursuit of this goal is to be guided by the following specific assumptions: 
 
• Fuel efficiency will be measured in terms of miles per equivalent gallon of gasoline. If an 
alternative source of energy is used, the goal will be miles per British Thermal Unit
3
 
(BTU) equivalent of a gallon of gasoline (or 114,132 BTUs). (Also, it is assumed that 
use of an alternative energy source will require separate government/industry activities 
and direction regarding BTU measurement and infrastructure challenges.); 
• Vehicles will satisfy Tier II emissions standards at the default levels of 0.125 g/mile of 
hydrocarbons (HC), 1.7 g/mile of carbon monoxide (CO) and 0.2 g/mile of oxides of 
nitrogen (NO
x
) at 100,000 miles while complying with other Clean Air Act 
requirements; 
• The target vehicles will meet the efficiency improvement goal while meeting present and 
future Federal Motor Vehicle Safety Standards (FMVSS); 
• Target vehicles will be at least 80 percent recyclable, up from the 75 percent industry 
average today 
• A concept vehicle should be available in approximately six years and a production 
prototype in approximately ten years.  
 
 
The following characteristics are to be used in defining comparable 1994 family sedans: 
 
• Family sedan function, i.e., capable of carrying up to six passengers with a comfort level 
equivalent to that of the 1994 Chrysler Concorde, Ford Taurus, and Chevrolet Lumina, at 
a fuel efficiency of up to three times the average of 26.6 mpg (unadjusted combined 
metro highway based on Federal Test Procedure) or 26.6 miles per 114,132 BTUs. Three 
times this efficiency is 80 miles per 114,132 BTUs; 
• Acceleration from 0-60 mph (0-100 km/h) in 12 seconds (at curb weight, with 300 lbs. of 
passenger load and a full fuel tank). 
• Luggage capacity of 16.8 cubic feet or 475 litres and load-carrying capacity equivalent to 
that of the comparable 1994 sedans (load carrying capacity includes up to six passengers 
with full fuel tank and 200 pounds of luggage). 
                                                           
3
  1 BTU = 252 cal = 1054 J 
 8
• Operating metro-highway range of at least 380 miles (610 km) on the 1994 Federal Drive 
Cycle. 
• Equivalent to comparable 1994 family sedans in all of the following aspects:  
- Performance in all areas, including acceleration, cruising speed, gradeability, and 
driveability at sea level and at higher altitudes 
- Ride and handling  
- Noise, vibration, and harshness control 
- Customer features and options, including climate control and entertainment packages 
- Total cost of ownership (with non-preferential tax treatment on a BTU basis) 
adjusted for economics 
- Minimum useful life of 100,000 miles (160,000 km) with comparable or improved 
service intervals and refuelling time  
- Capable of being easily produced for export and sale in major world markets. 
Fig. 3  Schedule for PNGV Technology Selection 
 
 
1.4 The PNGV Technology Challenge 
 
The distribution of energy in a current mid-size (family sedan) vehicle is shown in Figure 
4. Note that in this typical vehicle system, only 12.6 percent of the energy turns the wheels 
after engine, standby, driveline, and other losses on the U.S. Federal Urban Test Cycle. 
Major advances must be made in several technologies simultaneously in order to achieve an 
80 mpg vehicle. Improving the fuel economy requires a three-pronged approach: 
1) Convert energy more efficiently, 
2) Implement regenerative braking to recapture energy, and 
3) Reduce the energy demand from the vehicle. 
 
Fig. 4  Energy Distribution in a Mid-Size Vehicle 
 9
 
A parametric model has been used to show the various ways in which these approaches 
could be combined to achieve the 80 mpg goal. Figure 5 illustrates the approximate "design 
space" for possible alternative approaches for achieving the goal. 
Fig. 5 Achieving "3X" Fuel Economy Requires Major 
Improvements in Powertrain and Vehicle Characteristics* 
*All paths include: 90 percent efficient energy storage, 76.5 percent efficient driveline, 20 
percent lower rolling resistance, 30 percent lower accessories loads 
 
The design space has both theoretical and practical limits. On the basis of thermal 
efficiencies that are practical to achieve with various heat engines, the targeted fuel economy 
may not be attainable through engine improvements alone. The needed thermal efficiency 
ranges from approximately 40 percent to 55 percent, which is twice that of today's engines. 
Even with advanced fuel cells, which have higher potential efficiencies than heat engines, 
other vehicle improvements are likely to be needed. 
Even with improved engines and lighter vehicles, models show that an efficient 
regenerative braking system must be implemented to recover, store, and reuse energy 
currently lost by braking. This approach will reduce the amount of energy that must be 
converted from fuel, which is normally the most inefficient step of the energy cycle. 
In addition to improved power converters and regenerative braking, attainment of the fuel 
efficiency goal will likely require a 20 percent to 40 percent reduction in the mass of today's 
baseline vehicles. This will require more than simple refinements to today's frame and body 
construction, and calls for the introduction of entirely new classes of structural materials. 
Advances must also be achieved in several other vehicle technologies. Although each of 
these will contribute less to the overall system goal, their combined contribution may be 
significant. These technologies include reduced aerodynamic drag, reduced tire rolling 
resistance, and more efficient mechanical and electrical components. 
In order to achieve Goal 3, the PNGV Program Plan describes the research and 
development needs in the technology areas leading to improvements in the vehicle and 
propulsion system. These technologies include 
 
• Advanced Lightweight Materials and Structures 
• Energy Conversion Systems 
- Lean burn internal combustion engines 
- Gas turbines 
- Fuel cells 
 10
- Hybrid propulsion systems 
• Energy Storage Technologies 
- High-Power batteries 
- Flywheels 
- Ultracapacitors 
• Energy Efficient Electric Systems 
• Waste Heat Recovery 
 
In March 2000 the big three US auto makers demonstrated their latest hybrid prototypes 
to US Vice-President Al Gore. The 
test cars have been developed under 
the PNGV effort. GM's Precept 
4
concept car (figure on left) has 
reached the PNGV goal. One version 
of Precept is powered by a small 
diesel engine and an electric motor, 
which would get 80 miles per gallon 
gasoline equivalent (2.94 liters per 
100 kilometers), and a second Precept 
has a hydrogen fuel cell that gets 100 
miles per gallon. Concept cars 
developed by the two other auto 
makers have come close to the 80mpg target. All use a combination diesel-electric hybrid 
engine, more aerodynamic designs and lighter materials. GM said it planned to produce 
hybrid trucks, cars and buses. For a full-size truck, the target will be 15-20% better fuel 
economy [Bursa, 2000]. 
With gas prices now 
averaging more than $1.55 a 
gallon, Gore said getting more 
fuel-efficient vehicles to 
consumers was paramount. 
The US Government has 
included $14 million in its 
$256m budget request for 
PNGV in year 2001 to fund 
additional research into SUV 
economy, and Gore also urged 
the US Congress to pass the 
administration's $3,000 
proposed tax credit for 
purchasers of high-mileage 
vehicles such as electric cars 
now on the market. 
 
                                                           
4
 The GM Precept features the lowest drag coefficient (Cd = 0.163) ever recorded for a five-passenger, four-
door family sedan. The dual-axle regenerative, parallel hybrid propulsion system of the Precept features a 35 
kW three-phase electric motor driving the front wheels and a three-cylinder, 1.3-liter turbo diesel engine (40 
kW/54 hp) with common-rail direct-injection driving the rear wheels along with a multi-purpose unit. 
 11
 
1.5 Emissions from vehicles 
 
Autos release a variety of chemicals into the air that have been determined to cause or 
contribute to respiratory diseases, including bronchitis, emphysema, pulmonary fibrosis, and 
asthma.  
In the past, the majority of harmful chemicals, or pollutants, released from autos have 
been combustion products exiting the tailpipe. As auto manufacturers continue to decrease 
these pollutants, gasoline or other fuel evaporative and refuelling emissions become a more 
significant portion of the overall auto pollution equation. 
Primary auto-related pollutants include: 
 
• Volatile organic compounds (VOCs) are a group of hydrocarbon (HC) based chemicals 
from both tailpipe and evaporative sources. Some of these compounds have direct health 
risks that range from respiratory irritation to cancer. Levels of most of the confirmed or 
suspected carcinogenic substances are regulated. All VOCs contribute to some degree to 
the urban problem of smog. In the presence of ultraviolet light, VOCs react with nitrous 
monoxides (NO) to form ground-level ozone (O
3
) and photochemical smog (including 
NO
2
). High levels of ozone and urban smog have been shown to cause a variety of 
temporary and permanent respiratory ailments and related health problems including 
asthmatic attacks and emphysema. 
 
• Nitrogen oxides (NO and NO
2
, often referred to as NO
x
) result from high-temperature 
combustion reactions and, as noted above, contribute to the formation of photochemical 
smog. Nitrogen oxide (NO
x
) emissions are a major factor in international environmental 
problems. Early year 1999, the European Commission proposed an ozone alert standard 
for major cities of 60 parts per billion (ppb) over an eight-hour period. Nitric oxide (NO) 
is a colorless gas that is converted in the atmosphere to yellowish-brown nitrogen 
dioxide (NO
2
). NO
2
 can cause adverse human health effects, including bronchitis, 
pneumonia, lung irritation and increased susceptibility to viral infection. Animal studies 
indicate that intermittent, low-level NO
2
 exposure also can induce kidney, liver, spleen, 
red blood cell and immune system alterations. Furthermore, NO
x
 reacts with oxygen and 
other components of air to form nitrates, which can coalesce into fine particles. Study of 
collected PM-2.5 (particulate matter with an aerodynamic diameter of 2.5 microns or 
less) suggest nitrates make up more than 10 percent of the mass of fine particulates in 
some world regions. NO
2
 also acts with SO
2
 and water vapour to form acid rain. Acid 
rain has been shown to cause depression of photosynthesis and defoliation of trees, 
damage to fish and other aquatic life, and corrosion of building materials, particularly 
limestone.  
 
• Carbon monoxide (CO) is colorless and odorless tailpipe emission that results from 
incomplete combustion of the fuel in an internal combustion engine. A volumetric 
concentration of 0.3% can result in death within 30 minutes due to its ability to displace 
oxygen in the blood. Lower concentrations of CO can cause headaches, dizziness, and 
nausea. 
 
• Particulates, particularly those solids such as ash which are smaller than 10 micrometers 
in diameter (PM-10), pose a serious health hazard due to their potential to be inhaled and 
accumulated deep within the human lung. PM-10 is primarily an issue with compression 
 12
ignition (diesel) engines. The term "particle" refers to the airborne state as in the exhaust 
gases, where the particles are individual units and the term "particulates" refers to 
particles collected on a filter. 
Solid carbonaceous particles in the size range between 10 and 20 nm are formed in the 
combustion chamber [Kittelson, 1998]. The high number concentration before the outlet 
of the combustion chamber causes quick agglomeration. This is the process of particles 
adhering together after random collisions with each other. In this way, particle number is 
decreased but particle size is increased, while particle mass remains unchanged. The 
agglomeration rate, and hence the rate at which the number of particles decrease is 
directly dependent on the number of particles. The fewer the particles, the lower the 
agglomeration rate. Therefore, the rate decreases as the particles move through the 
vehicle’s exhaust system. 
During the cooling in the exhaust system condensation processes take place. Vapour 
condenses on the existing particles and new, volatile, droplets can be formed by 
nucleation. The vapour consists of unburnt lubricant and fuel, products of incomplete 
combustion and sulphuric acid, or sulphates dissolved in water. The formation of new 
volatile droplets depends on the amount of the gaseous species, the cooling and the 
dilution conditions after tail-pipe emission. The sulphur in the fuel is considered to be the 
main source of volatile droplets. 
In this way, exhaust particles leaving the tail-pipe consist of highly agglomerated solid 
carbonaceous material with adsorbents on particle surface. The agglomerates may carry a 
condensed layer of hydrocarbons and/or sulphur compounds on their surfaces and may be 
mixed with volatile hydrocarbon/sulphur or sulphuric acid droplets and ash, i.e. mineral 
fuel constituents. The final agglomerated structure can vary from loose flaky structures to 
compact forms [Skillas, 1997; Huang, 1993; Burtscher, 1992]. Due to this wide spectrum 
of possible structures, the relation between particle size and mass is not known a priori 
and is a subject of basic on-going research. 
Exhaust particles will undergo further dramatic changes after tail-pipe emission into the 
atmosphere by the processes of agglomeration, condensation and further chemical 
reactions on the particle surface. The emission conditions in real traffic situations are 
undefined and unknown. Extrapolation from the tailpipe emission to final particle size 
and number in the atmosphere is presently not possible. 
The attempts to decrease emissions led to the development of "advanced" Diesel engine 
concepts, which are high pressure (> 700 bar) direct injection systems. 
In recent years, the emission of fine particles has become an important topic in 
environmental, political, scientific, and public discussions. Some studies [e.g. Dockery, 
1993] have been focused on the relation between particle size and adverse health effects. 
Some findings indicate that these health effects could be due to very small particle sizes 
and their number concentration. Although particle mass is limited and measured, the 
relationship between number and size of very small particles in exhaust gases to 
particulate mass is still unknown. Recent discussions have tended to associate 
"advanced” Diesel engine technology with an increase in the number of fine particles, in 
spite of reduced mass emissions compared with "conventional" Diesel engines. To date, 
this statement has not been substantiated. 
 
• Carbon dioxide (CO
2
), unlike the other pollutants, will occur even in a perfect 
combustion (Reac. 1). The emission of CO
2
 is therefore directly related, in a linear 
fashion, with the engine's consumption of a given fuel. In the same way as oxygen or 
nitrogen, CO
2
 is a harmless gas which is naturally present in air. Although not a toxin, 
and although not yet subject to legislation, there is increasing concern that combustion of 
 13
fossil fuels is resulting in significant increases in global CO
2
 levels. This rise in CO
2
 may 
be causing shifts in weather patterns due to changes in reradiation of thermal energy 
from the earth’s surface to space. This so-called "greenhouse effect" global warming has 
led to international pressure to limit fossil fuel use. The USA currently produces more 
than twice the CO
2
 per capita as Europe or Japan. 
 
In ideal combustion, the fuel would be completely oxidized and the only product of 
combustion would be CO
2
 and H
2
O as shown in the following reaction: 
 
4HC + 5O
2
  ⇒  2H
2
O + 4CO
2
    (1.1) 
 
Unfortunately, since the reactants include a variety of other gases present in the atmosphere, 
specifically nitrogen, it is difficult to provide exactly the correct amount of fuel and oxygen 
for ideal combustion. Also, other secondary reactions occur at the high temperatures and 
pressures during and immediately following combustion. 
 
• Volatile Organic Compounds (VOCs) are defined by the EPA (US Environmental 
Protection Agency) as follows:  
 
"A volatile organic compound (VOC) is any organic compound which, once released into the 
atmosphere, can remain there for a sufficiently long time to participate in photochemical 
reactions. Although there is no clear demarcation between volatile and non-volatile 
compounds, compounds that evaporate rapidly at ambient temperatures account for the 
main share of VOC. Virtually all organic compounds which can be considered as VOC have 
a vapour pressure >0.1 mm
Hg
 in standard conditions (20 °C and 1 atm)."  
 
Unburnt hydrocarbons contain a large proportion of methane, but because methane is inert, it 
is not classified as a VOC. In addition to higher hydrocarbons (ethane, propane, ...), 
oxygenated compounds (aldehydes, ketones, phenols, alcohols, nitromethane and esters), 
which have a higher reactivity, are also considered as a VOC.  
 
VOCs are primarily produced from fuel that has escaped from the main combustion reaction 
unburned. These emissions are produced from the following conditions:  
 
- quenching or cooling of the flame near the wall of the combustion chamber,  
- fuel trapped in crevices in the chamber (i.e. piston rings, gap between the engine head 
and block, shrouded area around spark plug and gap between piston and wall),  
- fuel vapours absorbed by the oil film during combustion and desorbed after 
combustion,  
- incomplete combustion due to excess exhaust gas recirculation, misfires or other 
similar occurrences.  
 
All the above result in unburnt fuel being passed through the engine into the exhaust. 
Benzene (C
6
H
6
) - an often monitored VOC emitted by vehicle exhausts - concentration 
levels are small compared with some other pollutants. It is a known carcinogen and an 
important risk factor in leukaemia in particular. The figure below shows the concentration of 
benzene measured in six European towns [Venice, 1999]. There is a clear increase in 
benzene pollution as you travel southwards across Europe. A number of variables must be 
taken into account to explain this difference, including, no doubt, traffic density and flows, 
the influence of climate and weather, lifestyles and the structure of the built urban 
 14
environment. There was another clear finding: benzene concentration levels are generally - 
and paradoxically - higher indoors than outdoors. In December 1998, the European 
Commission submitted a new proposal for a directive which, for the first time, will set a 
benzene concentration limit. The proposed 
objective is to reach what is seen as a 
precautionary limit of 5 µ g/m
3
 urban air by 
2010 [RTD info 23, 1999]. 
 
• Nitrogen Oxides (NO
x
) represent the NO 
and NO
2
 emissions grouped together. The 
NO emission dominates this mix. For 
most operating conditions the NO
2
 
percentage is very low (see NO
2
 below 
for specifics).  
 
i) NO production is based upon the fuel-to-air 
(F/A) ratio, the fraction of burnt gases from exhaust gas recirculation (EGR), residual burnt 
gases left in the combustion chamber from the previous cycle, and the engine timing. The 
F/A ratio is important because for significant NO to be formed, there must be excess free 
oxygen present. This can only occur if too little fuel is mixed with the air, resulting in excess 
air remaining after the combustion reaction. The primary reactions for NO production are as 
follows: 
 
N
2
 + O
2
  ⇒ 2NO      (1.2) 
 
and according to the extended Zeldovich model [Zeldovich, 1947] 
 
N
2
 + O  ⇒ NO + N     (1.3) 
N + O
2
  ⇒ NO + O     (1.4) 
N + OH  ⇒ NO + H     (1.5) 
 
Assuming there is excess oxygen present, the rate of formation of NO is then dependent 
upon the temperature. NO
x
 production will increase as the temperatures in the combustion 
chamber increase. The burnt gas fraction and ignition timing are the two major factors 
determining temperature in the combustion chamber. As the burnt gas fraction is increased, 
the combustion temperature is decreased, resulting in a decrease in the NO production. A 
decrease in timing advance will also lower the cylinder temperature and decrease the NO 
production.  
 
ii) NO
2
 is produced in the flame, but is immediately reconverted to NO as shown in the 
following reactions:  
 
NO + HO
2
  ⇒  NO
2
 + OH     (1.6) 
NO
2
 + O  ⇒  NO + O
2 
    (1.7) 
 
Equilibrium equations show that NO
2
 concentrations should be negligible unless the above 
equations are quenched prior to completion. This can occur in compression ignition (i.e. 
diesel) engines running at low speed and low loads. This can also occur in spark ignition (i.e. 
gasoline) engines that idle for extended periods of time. In both cases, the flame 
 15
temperatures are similar, but the surrounding gases are much cooler, allowing the above 
reactions to be easily quenched. In diesel engines low speed and load can raise the NO
2
/NO
x
 
percentage from negligible to 30%.  
 
• Carbon Monoxide (CO) is produced as an intermediate in the production of CO
2
. CO is 
produced through the following series of reactions:  
 
 
RH  ⇒  R  ⇒  RO
2
  ⇒  RCHO  ⇒  RCO  ⇒  CO   (1.8) 
(R represents the hydrocarbon radical) 
 
CO
2
 is then produced at a slower rate through the following oxidation reaction:  
 
CO + OH  ⇒  CO
2
 + H     (1.9) 
 
In rich mixtures, concentrations of CO are a result of incomplete combustion and are directly 
proportional to the fuel-to-air (F/A) ratio. As the F/A ratio increases, the CO concentration 
increases. The equilibrium of the following reaction can provide an approximation of the 
concentration of CO gases:  
 
CO + H
2
O  ⇒  H
2
 + CO
2  
   (1.10) 
 
For temperatures of 1600 - 1700 K, the equilibrium constant is usually between 3.5 and 3.8. 
CO is mainly the result of rich mixtures and only present in much smaller concentrations in 
lean mixtures.  
 
• Particulate Matter (PM) is small particles in the exhaust stream. The mechanics of 
formation differ for the spark ignition (i.e. gasoline) and compression ignition (i.e. 
diesel) engine.  
 
- In the spark ignition engine there are three main causes of particles: lead, sulfur and 
rich mixture. In modern fuels that are unleaded and have low sulfur content, the lead 
and sulfur are not significant contributors to PM emissions. Particles from a rich 
mixture are also negligible in well-tuned engines.  
 
- In the compression ignition engine, soot is the primary particulate and there are three 
reactions that control the level of PM emissions:  
 
i. soot is formed in the reaction of fuel and air during the first combustion phase  
ii. soot is formed from excess fuel in the burnt gases during the second combustion phase  
iii. the soot is oxidized after it is mixed with the oxygen-rich areas in the combustion 
chamber  
 
These three reactions act to form and oxidize the soot particles. Levels of soot production 
can also be affected by factors such as fuel droplet size during injection, pre-chamber 
ignition, and deposits on chambers walls.