WhiteBlue wrote:The "late" ignition is to make sure that you have sufficient evaporation due to the comparatively late injection. That is what they have found gives you the most efficient combustion. No, there is only one ingnition normally. Of course one can also go to laser based linear ignition, which is better but more expensive.
The reason why this is more efficient is the maximum use of the the fuel evaporation enthalpy for suppression of knocking. It means you can go leaner even at full power. At port ignition the evaporation happens in the ports and the cooling energy gets lost for the compressed charge. There is also fuel lost which remains on the walls of the intake system. As I have said before the direct spray guided injection gives a massive step in fuel reduction compared to port injection. At part loads it can be up to 20% and at full load I still reckon 5-10% .
To provide maximum torque and the lowest fuel consumption you have to ignite the charge so that peak cylinder pressure occur at 15-20 degrees after top dead center.
This is simply a question about piston movement vs. heat release rate. If you ignite the mixture so that the peak pressur occur later, you can't effectivly expand the gases and the torque produced during the power stroke decrease, and if you ignite the mixture earlier you will have too much pressure build up before top dead center where it will produce negative torque on the crank.
Spray guided stratified charge is also for part load operation, at high load, the fuel injection will occur during the intake stroke (after the exhaust valve have closed) with a homogeneous charge.
A later point of ignition will also require a richer air/fuel mixture to prevent excessive exhaust temperatures. That's why engine management systems are set to enrich the air/fuel mixture when the ignition is retarded. The retarded ignition itself reduce peak cylinder pressures and with that the risk for engine knock.
747heavy wrote:How about this?
Not really suitable for a modern direct injected racing engine where you want to avoid swirl which can lead to cylinder wall wetting. But this was the eightes when there was a bit of focus on just swirl. Then the focus shifted from swirl to tumble.
Of course, diesel engines are still swirl focused. So there you can find some similar solutions.