Why don’t some manufacturers make a compression-ignition
petrol engine by simply increasing the compression ratio?
DAVID McKINNEY – By e-mail
Such an engine would not even start, let alone run. The two combustion processes are very different, which is why the two engine designs are so different.
The petrol engine combustion process takes place in a combustible mixture as the result of a flame initiated by a spark.
This flame travels outwards from the spark in the shape of a ball. The result is that the end-gas (the gas mixture in front of the flame) increases in temperature and pressure, causing it to develop a tendency to self-ignite or detonate before the flame gets to it, causing shockwaves that can destroy an engine. This tendency can be curbed by using highoctane fuel or lowering the compression ratio.
The latter course will reduce an engine’s efficiency so that the modern tendency is to run with compression ratios close to 12:1, which is about as far as a petrol engine design can go without the threat of self-destructing.
Diesel engine combustion takes place after the air inside the cylinder has been heated by a compression ratio of between 16:1 and 20:1.
The fuel is then introduced by injectors in the form of a fine spray. Each fuel droplet has to find some oxygen before it can ignite, with the result that diesel combustion occurs haphazardly all over the combustion area.
The big advantage is that since the engine only inhales air the end-gas cannot self-ignite due to pressure buildup, which is why a high compression ratio can be employed.
Mercedes-Benz has shown an engine to the press that combines the petrol and diesel cycle in one unit.
When cruising at normal speeds the engine runs on the diesel cycle, to get the best fuel economy, but when maximum power is demanded the engine switches to the petrol cycle, because this is where a petrol engine scores over a diesel. This engine has an undisclosed mechanism that changes compression ratio as required.
