Compatibility
Due to their high efficiency compression-ignition diesel engines are the leading power source in heavy-duty vehicles, because of their high efficiency. Today diesel engines are becoming more popular also in light-duty cars. Emission control devices and internal engine solutions have crucial effects on the exhaust emissions. Diesel engines are running on a lean mixture, which improves fuel consumption, but at the cost of increased nitrogen oxide emissions (NOx). NOx emissions are formed from nitrogen in the air at high temperatures. High particulate matter (PM) emissions are another problem of diesel engines.
Selective catalytic reduction (SCR) and exhaust gas recirculation (EGR) are the common technologies used for reducing the NOx emissions of diesel engines. EGR is an internal engine technology, whereas SCR is an exhaust after-treatment device using a reducing agent, such as ammonia or urea. With EGR some of the exhaust gas is returned to the engine cylinders, which lowers the combustion temperature and consequently NOx emissions. High EGR ratio may lead to problems with engine cleanliness, and particulate matter emissions may increase. Oxidation catalyst reduces volatile organic emissions. Particulate filters reduce efficiently particulate matter emissions.
Spark-ignition, gasoline fuelled engines are the leading power source of passenger cars. Spark-ignition engines are simple and cheap when compared to compression-ignition diesel engines. In addition, stoichiometric air-to-fuel ratio allows usage of three-way catalyst (TWC), which is capable of reducing carbon monoxide (CO), hydrocarbons (HC), and nitrogen oxides (NOx) emissions simultaneously and efficiently. A drawback of spark-ignition engines is their lower efficiency when compared to compression-ignition engines. Therefore fuel consumption of spark-ignition engines is higher than that of diesel fuelled engines both in energy and in volumetric terms.
Gasoline cars equipped with carburetor engines were available until the late 1980s. Today, spark-ignition engines are port-injection engines, mostly equipped with multi-point fuel injection (MPFI, fuel injected into the intake port). In the 1990s, direct-injection spark-ignition engines with higher efficiency and lower fuel consumption appeared on the market. Models using lean combustion with excess air were also introduced in the 1990s, but they soon disappeared from the market. Spark-ignition engines, whether in-direct- or direct-injection, are now based on a stoichiometric air/fuel ratio, and are equipped with TWC catalyst.
The exhaust emissions from spark-ignition engines using a stoichiometric air/fuel ratio can be efficiently controlled with a three-way catalyst (TWC). In TWC which carbon monoxide and unburnt hydrocarbons are oxidized simultaneously with the reduction of nitrogen oxides. With TWC even more than 90% reduction in engine-out CO, HC and NOx emissions is achieved, and emissions occur mainly at cold start or heavy acceleration. However, in some conditions TWC catalyst may generate ammonia and nitrous oxide emissions. TWCs operate efficiently only in a very narrow lambda window close to the stoichiometric air/fuel ratio and therefore TWCs cannot be used in engines running with a lean mixture, such as diesel engines. The benefit of a lean mixture would be improved fuel consumption, but at the cost of increased NOx emissions. Exhaust gas recirculation (EGR) is one of the common technologies used for reducing the NOx emissions of diesel engines, and it is also used in spark-ignition engines. For direct-injection spark-ignition cars, particulate matter emissions are high, and therefore particulate filters may become necessary.
Spark-ignition engines today are less sensitive toward fuel than older engine generations, and absolute mass emissions are low. However, at cold starts, heavy driving conditions, and at low temperatures, there may be large differences, absolute and relative, between fuels for all cars. In the past, carburetor engines were especially sensitive toward fuel, for example, drivability and vapor lock problems were experienced. Most of the gasoline-fuelled cars today can tolerate at least up to 10 vol-% ethanol in Europe and the U.S.