Timing and tuning

[Peter Cobbold]

Hi Tom,

Ah, the 8.5:1 head lacks squish. The combustion chamber is at full bore for a couple of mm above the piston crown so squish - or 'quench' to you? - is lacking. That reduces turbulence in combustion and that can lead to knock/pinking.

 

Higher octane fuel will not improve performance if the engine does not detonate on lower octane fuel.

If an engine pinks on lower octane then the spark has to be retarded and that can drop mep unacceptably. In that case the higher octane allows the correct spark timing to be restored and the max 'push' on the piston comes at the optimum degrees ATDC

 

HIgh octane race fuel can be slighly denser, 1%, than pump fuels so maybe that might give a tiny bit more economy. But usually power is limited by air swallowing and fuel flow is never a problem so higher density is not useful for power. As far as I know - its a complicated field I have not mastered - the volume of vapour from low and high octane fuels is the same for a given AFR, so air displacement by vapour doesnt change with octane. The only difference as far as I can tell is the high octane doesn't autoignite in the end gas readily - the chemistry of the fuel ensures that. And autoignition is aka pinking/knock/detonation.

 

I'm puzzled by the widespread suggestion that ethanol boils easier than petrol. Fuel vapour pressure is tightly regulated.

And ethanol is far less volatile than petrol too. Indeed E85 has that 15% petrol added to ensure the vapour space in the tank is saturated with petrol vapour - ethanol lacks volatility and without the 15% petrol the alcohol vapour will contain enough air to be explosive. The vapour space in a petrol tank stays saturated with vapour down to about -30C, excluding air. That's safe. While neat ethanol wont saturate the space and displace all air even at 18C - the tanks are hazardous, the slightest spark.....

cheers,

Peter

[Tim D.]

Peter,

Not sure I agree. The MU is essentially a metered pump whose output is defined by the shuttle stroke (governed by the pressure applied to the diaphragm) multiplied by the rotation speed ( governed by the crank rotation speed). Therefore in theory the MU output is the same whether there is load or not. You could in theory test the pump without the engine by just rotating the MU at desired top speed with the pressure input at atmospheric pressure..

The issue with testing with the unloaded engine is that the revs cannot be held at redline with the throttles wide open. Whereas on a rolling road (where load can be manipulated) or perhaps on a steep hill the engine can be held at redline..

Hope this makes sense :-)

Cheers

Tim

[Peter Cobbold]

Therefore in theory the MU output is the same whether there is load or not.

Tim,

That's most definitely not the case.

When the engine is run out of gear it is impossible to get the intake manifold anywhere near to atmospheric, due to the barely open butterlflies. The MU diaphragm experiences a high depression and reduces the shuttle-displaced volume to that required by the air flow - around 5 hp at 5000rpm.

 

Running at wide open throttle is possible on a rolling road, or on the road, and will allow the MU diaphragm to see atmospheric pressure, at any rpm. But that is the full load condition !! - and the shuttled volume will be maximal.

 

So the MU shuttle volume is highly dependent upon horsepower. At wot the horsepower generated will be in proportion to the shuttle volume multiplied by rpm. And that means the fuel flow depends upon horsepower.

And so does air flow. 'Of course' :)

 

Peter

Edited August 20, 2015 by Peter Cobbold

[Tim D.]

Hi peter,

Think we are actually saying the same thing. I was meaning that while theoretically possible to run the engine unloaded at WOT practically it would exceed the redline ( except where a rev limiter was present). Absolutely agree that if you run the engine at redline without a Rev limiter the throttles will only be slightly open.. And hence shuttle travel will be limited.

With a Rev limiter you can have wot and 5500 which are the inputs you want to the MU to test fuel flow at maximum usage. Of course this is no good for measuring afr as most of the fuel going into the engine will be unburnt.

Cheers

Tim