109 e3b against spitfire II

[Viper2000]

Originally Posted by Crumpp

http://www.automobilemag.com/feature...ion/index.html

Single point injection has no advantages over direct fuel injection at all. The Supercharger is on a completely separate circuit and the engine still receives all the benefits of supercharging with the additional benefits of direct injection.

The supercharger is driven by the engine.

If you reduce the power consumed by the supercharger then you increase the brake horsepower and reduce the SFC.

Supercharger power consumption is just W*Cp*deltaT, ie W*deltaH.

Supercharger isentropic efficiency is

deltaH[actual]/deltaH[isentropic]

In the case of the Merlin, this figure was about 70%.

For isentropic, adiabatic compression,

T2 = T1(P2/P1)^(gamma/(gamma-1))

Hence it's trivial to calculate the isentropic deltaT, and deltaH.

DeltaT and deltaH both get smaller if we reduce T1.

Injecting fuel upstream of the supercharger reduces the temperature by about 25 K due to the latent heat of evaporation of the fuel.

This reduces the temperature rise across the supercharger, which is equivalent to increasing its adiabatic efficiency.

Clearly this confers an advantage to engines which inject fuel upstream of the supercharger. Given the considerable difficulty associated with increasing the aerodynamic efficiency of compressors, this advantage is not insignificant.

Mixture distribution is going to be very good provided that the charge temperature is sufficiently high for complete evaporation to be ensured. This will basically always be the case at high powers because deltaT is 100 K or more; indeed intercooling & aftercooling start to become necessary once you've got a lot of supercharge.

These advantages vanish at low non-dimensional power settings. Cars spend most of their time at very low non-dimensional power settings, and therefore DI wins hands down most of the time, especially if you go for CI, in which case it's almost no-contest.

In the end, the nature of all engineering trade studies is that the devil is in the detail. The optimum is a strong function of engine size and duty cycle, and we just don't build the sort of highly supercharged, high power spark ignition engines for which single point injection is attractive these days.

To use an analogy, old amplifiers used valves and therefore tended to have large transformers & rectifiers to produce the high DC voltages which allowed them to function. Most modern amplifiers are solid state, and they don't need those high voltages.

This doesn't mean that high DC voltages aren't still a good idea for valve amplifiers; I've got a pair of hundred watt half stacks sat next to me which run in excess of 400 V DC and sound great. But probably 99% of modern amplifiers for domestic use are solid state and so if you just ask "are high voltages a good idea for amplifiers" then the short answer is "probably not".