17.9.
Engine Design and Modification
Many design details of an engine have a significant effect on pollutant emissions. These are similar to many variables in a very complicated equation. Keeping this in mind it should be understood that the final design of an engine is a compromise between conflicting interests. The major areas of interest are as presented below.
Combustion Chamber Design.
The major hydrocarbon emissions are due to the unburnt fuel in contact with the combustion chamber walls. Hence the surface area of the combustion chamber walls should be as small as possible and with the minimum complicated shape. A theoretical ideal is a sphere, which is not feasible in practice. A good swirl of the charge is important, as this facilitates better and more
rapid burning. Perhaps more important is to ensure a good swirl in the region of the spark plug. This provides a mixture, which is easier to ignite. The best position for the spark plug is the centre of the combustion chamber, as this reduces the possibility of combustion knock by minimising the distance the flame front has to travel.
Compression Ratio.
In general the higher the compression ratio, the higher is the thermal efficiency of an engine, so that better its performance and fuel consumption. The two major drawbacks of higher compression ratios are increased exhaust emissions and the increased tendency of the engine to knock. At higher temperature production of nitrous oxide is greater. Also at higher temperature the self ignition of charge of fuel and air is more likely, causing combustion knock. Countries having stringent emission regulations such as the USA and Japan have tried for some time to develop lower compression engines. However, changes in combustion chamber design and the introduction of four valves per cylinder, together with accurate electronic control of engine and other methods of controlling emissions, have made it possible for the increase in the compression ratios over the years.
Valve Timing.
The effect of valve timing on exhaust emissions can be quite significant. One of the main factors is the amount of valve overlap. This is the time period in which the inlet valve has opened but the exhaust valve has no yet closed. The duration of this phase determines the amount of exhaust gas left in the cylinder. This has a considerable effect on the reaction temperature, which decreases with increase in exhaust gas, and hence has a direct effect on the emissions of nitrogen oxides. The main conflict is that, at higher speeds, a longer period of inlet opening although increases the power developed, but also causes a greater valve overlap, which at idle can greatly increase emissions of hydrocarbons. This has, however, led to the successful electronic control of valve timing.
Manifold Designs.
Flow of gas within the inlet and exhaust manifolds is quite complex. The main reasons of this complexity are the transient changes in flow, due to changes in engine speed as well as in the pumping action of the cylinders. The change in pumping action causes pressure fluctuations in the manifold. If the manifolds and both induction and exhaust systems are designed to reflect the pressure wave back just at the right time, great improvements in volumetric efficiency can be achieved. Many vehicles are now fitted with adjustable length induction tracts. Longer tracts are used at lower engine speeds and shorter tracts at higher speeds.
Charge Stratification.
If the charge mixture can be inducted into the cylinder in such a way that a richer mixture is in the proximity of the spark plug, then overall charge in the cylinder can be much weaker. This can provide great improvements in fuel consumption but the production of nitrogen oxides can still continue. The introduction of direct injection of mixer is a good example of the charge stratification.
Warm Up Time.
A large percentage of the emissions produced by an average vehicle are generated during the warm up period. Suitable materials and care in the design of the cooling systems can greatly decrease this problem.
