Quote:
Originally Posted by Topless
Ummm, not if it's running gasoline. 18:1 is way to lean to be safe for full power applications and would not be making good hp numbers. I suspect the exhaust sensor was fouled. Ultra lean burn on direct injection motors is useful at idle or cruising down hill, not making max power on a dyno.
Gasoline direct injection - Wikipedia, the free encyclopedia |
I never said that his number was accurate. What I said is that it's unlikely that his engine would not run at an AFR of 18:1. If it was a carbed engine then yes (because of the uneven fuel distribution that is typical of carbed engines) 18:1 would be difficult to attain without rough running or missing.
And yes, I do much leaner than stoichiometric (14.7:1) all the time when running a 360CI, magneto timed (fixed @25BTDC) air-cooled flat 4 with the throttle wide open. Can you guess the application?
What do you think happens to the combustion event as the mixture goes lean of Stoic? Given that Stoic is the theoretical perfect mix of O2 to fuel. Is having having more O2 some how more combustible? The fastest burn of the most fuel happens just a tad rich of stoic as does the highest internal cylinder pressure thereby giving the lowest detonation margins (not good). At full throttle, this is not where you want the mixture to be. You want to be richer or leaner, but since we are limited by atmospheric pressure with N/A engines and turbine inlet temp with turbocharged engines, it's far easier and practical just to puke gas through it to slow (relatively speaking) the combustion event and cool the exhaust gasses thereby increasing detonation margins.
BTW the whole reason behind direct injection is to improve fuel distribution to allow very lean mixtures. In theory this will eliminate the need for a throttle body and the suction losses associated with partial throttle operation. Imagine sucking through a straw with partially cocked butterfly valve; that's what your engine is doing whenever it's not at full throttle. With lean burn engines, the engine management system will just reduce fuel flow, the mixture will get leaner and the engine will produce less power, no suction loss from choking off the intake. Another benefit is little to no unburned hydrocarbons, meaning we may have catless road going cars in our future.
I could go on, but I'm digressing! So to come back to the original point, an injected otto cycle engine should run still run at an efficiency ratio of 18:1, whether or not it will grenade in the process depends on throttle position, timing and compression ratio. The ratio in and of itself is not a death sentence... 14.7 to one is actually a hotter, higher pressure place to run