Diesel engines are HIGH compression turbo's 20:1 and stuff
That's how they work though, the pressure is needed to ignite the fuel.
I've been doing some research into engine thermodynamics and I've come up with some plots of what changes when you turbocharge a factory NA engine. Thought I'd post them here with my thoughts on it for discussion. As always, I welcome people telling me if they think I'm wrong.
Anyone who's ever sat a thermodynamics class will be familar with a PV diagram for an otto cycle. If not, some links
here and
here
Here's one comparing two engines with B18C geometry, both 11:1 compression, 45° MBT ignition timing, but one force fed 0.5bar to 150% VE. All plots taken at 7200rpm and 13.5AFR.
Here is the torque output comparison for one cylinder over one engine cycle.
Clearly the boosted engine is producing a huge amount more cylinder pressure at peak (over 1300psi), which while it makes a huge amount of torque, 234lbft vs 156lbft, it would most likely destroy the engine pretty quckly. The NA engine peaks at just under 900psi, so we'll take this as the safe limit for cylinder pressure.
Below, I've reduced the ignition timing back to 28.5°, the peak cylinder pressure is back at 900psi, but importantly the average pressure has risen from 210psi to 266psi, resulting in 213lbft. This would probably run pretty well.
Next, we'll up the boost to 1bar, for 200% VE. This requires the ignition to be dropped back to 16° to keep the peak cylinder pressure under 900psi.
I'd imagine this would not run all that well, but I could be wrong. The hugely retarded timing is obvious just past peak as the pressure drops before the fuel burn really gets going. It makes 244lbft.
Now, lets leave that engine as it is, and take the other one up to 200% VE, but with a 9:1 compression:
Ignition set to 25.4°, torque now at 264lbft. Lots more area visible, and the torque trace shows a gain pretty much everywhere unlike the previous ones which were a drain on torque on the compression stroke. This to me seems a better design for this level of boost.
Food for the discussion...