by PoetsRevenge » Sun Oct 22, 2017 8:39 pm
You're probably past this point but the hard hurdle really wouldn't be the fuel metering in such a situation.
The hard part is getting the fuel to actually burn at a fast enough rate to produce any power. Diesel is exceptionally difficult to ignite in a rapid explosion like gasoline. Diesel engines typically use 20:1 compression and then add 2 bar of boost on top of that to net effective pressures in the 40-50bar range for a good reason. It simply takes that much heat and pressure to get the burn time down for the fuel so that it gets rapid expansion of both the fuel, and the air it consumes during combustion. The hard part in a spark ignited gasoline engine is that the initial pressures are low. The fuel isn't going to combust quickly without a very long lead in time on the spark, so that the initial sparked fuel burning push the pressures up to the point where the rest fires simultaneously instead of just slow flame propagation. Now if you're talking a multi-fuel engine with high compression and a turbo charger already, much of this can be mitigated by simply upping the boost pressure to the point where compression is in the right range, then firing the plug around 20deg BTDC and adjusting pressure until optimum output. Such an engine should still start relatively high on compression, have direct injection, and simply run lower pressures (boost) with late ignition timing on other fuels. In effect, if building from scratch to be tolerant, one should actually build a diesel engine first, then lower compression just enough they can make it manageable on gasoline as well, not the other way around.
WIth direct injection you can run 16-18 bar on gasoline, it won't be ideal, but it does work, just hang the intake cam timing very late on gasoline, so that the effective (dynmaic) compression is closer to 12bar, and fire the plug about 20btdc at 3k for a moderate (3.5"bore) motor, and about 5deg or so at idle with a very slow progressive advance.
I would do such a project on something with DOHC's and variable valve timing, and i would probably start with the variable intake timing off a tooth or so, then reprogram for the new sweep. This way the can would run "strait up" timing only when at full advance, and only use that on diesel fuel. I would then retard it about 50degrees when running gasoline. I'd simply adjust turbine boost pressures for each fuel to get optimum efficiency and power output (max peak pressure it will tolerate)
Such a build wouldn't actually be that hard to engineer and machine for, if you used a newer DI,DOHC motor like say the new ecoboosts.
Sorry for reviving an old thread, just saw the subject and thought i'd chime in to offer a bit, and maybe spark your interest back up.