“But a 1.4 turbo gas engine should be able to produce 140 bhp but I suspect it would need a big turbo and be not nice to drive…”
VW’s blown 1.4 gas TSI produces 170 bhp, and I understand it is nice to drive. But, is blowing gas engine such a nice idea? We have to remember that turbo charging (as well as supercharging) is a method to burn MORE gas. Therefore, it is wrong to state that the engine sips gas like a 1.4 and produces power like an NA 2.0. The advantage of turbo charging a small gas engine is possibly lighter weight and smaller package even with the added weight and bulk of the turbocharger than an NA engine of comparable power especially when we are comparing I-4 and V-6. (BMW’s I-6 335i engine is blown possibly because there is no more room to expand bore and/or stroke for more power.)
But there are disadvantages:
1. Increase in pressure inside combustion chamber. If the boost pressure is 1000 hPa, for example, a 10:1 compression ratio (geometrically) engine becomes in effect a 20:1 engine, which certainly causes spontaneous combustion or knocking unless you do something (see notes below).
2. Exhaust plumbing must be compromised (in the area of back pressure and sonic pulse tuning).
3. More waste of energy as heat (Engine runs hot).
4. Mechanical components must withstand the increased heat.
Maybe more…
Notes:
A) Squirt more fuel into the combustion chamber than the available oxygen can burn it. (poor fuel mileage and increase in raw gas, hydrogen and/or carbon in exhaust)
B) Retard spark timing. (poor fuel mileage, reduced power)
C) Retard closing of intake valve after piston goes into compression mode (Miller cycle), thereby lowering effective compression ratio. (reduced power, but increased efficiency)
D) Inject fuel into the combustion chamber when piston reaches its top and let it burn quickly either by spark plug or by compression.
