riff_raff wrote:mach11,
A diesel engine can operate happily with an A/F ratio of 45:1. But a spark-ignited engine (like in F1) wants something very close to stoichiometric (approx. 14.7:1) for best power. An very lean A/F ratio of 45:1 in an SI gasoline engine would result in severe detonation and unstable combustion.
A word of advice: Wikipedia still promotes the widely discredited myths about AGW, so you should probably find yourself a better technical reference than Wikipedia.
Regards,
riff_raff
With a 45:1 air fuel ratio there probably wouldn't be a burn at all.
And you don't have such big problems with knock running very lean mixtures. With a lean mixture the fuel will burn very slowly, if at all, and that will significantly reduce the peak pressures in the cylinders.
The leaner you go, the more slowly it burns, which also will reduce engine efficiency and output. That's why lean running direct injected engines use a stratified charge where a near lambda 1 mixture is achieved just around the spark plugs. That way you can use a lean overall mixture, but with a faster more normal combustion.
F1_eng wrote:Ratios below 0.8 are not seen on modern racing engines, that is very rich.
The reason you will typically see the ratio decreasing with higher speeds typically for a turbo is because the charge temperature also increases. One way to stop detonation at elevated charge temperatures is to overfuel to cool the temperature.
This is one of many problems that downsized, highly turbocharged economical engines of the future will have to deal with: there is no use in downsizing the engine for economy then have to inject more fuel to deal with the high inlet temperatures because of the high boost pressures.
I believe that using ethanol as a fuel could be a very effective way of overcoming this issue. Not sure if any manufacturer is looking at this method, have been out the engine development loop for a couple of years and developments come thick and fast.
I am not going to post much on this topic as I don't have time now but it's definetely interesting.
Passenger car engines are normally run at part load, they spend very little time at full load. So the fuel efficiency at full load is not as important as the fuel efficiency at part load. So that's why you downsize engines, to improve part load efficiency.
Also, modern turbo engines are capable to run quite lean at full throttle. This is due to two reasons. The use of high temperature materials and designs in components like exhaust valves, exhaust manifolds and turbocharger turbines. The second is the use of temperature based fuel enrichment. The ECU calculates the exhaust temperature and only inject extra fuel for cooling when needed. That way it's possible to do short high load accelerations without injecting extra fuel for cooling.
mach11 wrote:zmej,
regarding numbers of a modern f1 engine... the air-fuel mixture is in the ratio of 12.5:1 or 13:1....
my question is if the compression ratio of the engine is increased can a leaner air fuel mixture be used to obtain the power which is produced currently???
I would put most circuit racing engines in the lambda 0.85 to lambda 0.95 for peak power. Closer to the former for indirect injected engines and the latter for direct injected engines.
Shaddock wrote:If F1 went direct injection, then you could increase compression ratios and achieve a more efficient burn.
No, the compression ratio of a F1 engine is not knock limited. And a higher compression ratio does not mean a more efficient burn, a higher compression ratio only means that the engine can make better use of the heat released by the burn.
But the compression ratio of a F1 engine is limited by geometry. If you want to increase the compression ratio, either the burn or the volumetric efficiency will suffer which will reduce the output of the engine.
F1_eng wrote:It is this "injecting synchronously" that is the biggest issue with regards to Formula 1 engines. You have a very narrow time window to get all the fuel injected, with current injector technology, you can't get the required ammount in to the cylinder at 18,000rpm.
You can, it's just that you will have to develope a very high pressure gasoline injection system. Gasoline direct injected engines rarely use pressures higher than 150 bar, to run at 18,000 rpm expect to at least double or triple that.
riff_raff wrote:An F1 engine runs 99% of the time at full load WOT conditions. So an equivalence ratio near stoichiometric or slightly rich is desirable for best power. An injection point as far up the inlet runner as possible is used to maximize the charge cooling and densifying effect of the fuel latent heat.
DI would not be beneficial from the standpoint of achieving a homogeneous intake charge mixture, which once again is beneficial for best power. DI tends to produce a stratified intake mixture. A stratified mixture can be put to good use if lean combustion is desired, such as part load, high speed operation. But a stratified lean mixture would not produce the best power. In fact it would likely result in detonation under full load WOT conditions unless the ignition timing is retarded appropriately.
Where cost is not a factor, production engines employ both port and direct injection. With each system being utilized under specific conditions:
riff_raff
Direct injection is beneficial for both power and fuel consumption (which translates into a weight advantage), this based on what have been achieved with other high performance engines (such as several current Le Mans engines and even aircraft engines back in the 1940'ies). However, to take full advantage of direct injection, the engine needs to be designed for it.
The use of both direct and indirect injection is probably a compromise, there can for instance be issues with injecting the required amount of fuel at high loads and speeds using only direct injection. Although that can be solved with a properly designed fuel injection system and a properly designed engine, combining both is probably a stop-gap measure.
