[Vwdiesel] Burn ratios, compression and gas turbines

Val Christian val at swamps.roc.ny.us
Wed Jul 21 00:13:49 EDT 2004

>   "At idle, the air-to-fuel ratio may be greater than 100:1; even at top 
> speed, the mixture never approaches the comparatively rich stoichiometric 
> air-to-fuel ratio of approximately 14.5:1 that is common to spark-ignition
> engines."  (Bentley Manual: Fuel system - Diesel section.)

	For jet fuel (JET-A) the ratio is something like 18:1.

	Gasoline has a 14.something ration.

>   It looks more like if you want to lean out your engine, slow down.  
> As long as you're not at the limit of what the pump's set at, turning 
> the screw back will only limit your maximum acceleration.  It 
> won't make things any more lean at cruise.

	A Volvo (or other engine) with SU or Stromberg carbs is
	a variable venturi, but constant vacuum engine.  
	The VW IDI diesel is a constant volume engine, at a given 

	Some turbines run constant volume, when the shaft is direct 
	to the drive.  When there is an intermediate turbine, 
	the the turbine will change volume a small amount, as there
	will be a differential between the compressor and output
	shafts.  Intermediate turbines are neat, because their 
	torque is highest at low output shaft speeds.  Unfortunately,
	they are not the most efficient transmission.

	Also, gas turbine engines tend to perform most efficiently in
	a narrow power output range.  Hence applications like planes 
	and some trains, and power generation facilities.  But their
	power to weight ratio is great!

	For the curious, compression ratios can reach 30:1.  Oh, the 
	noise...that's because a typical turbine has exit velocities 
	of Mach 2.  High bypass turbofan engines, "cushion" the exhaust
	with a blanket of subsonic air.  That helps keep the acoustic
	emissions down.

	When I was younger, I figured we'd see streamlined cars for 
	intercity commuters, which used a 40hp gas turbine.  Obviously
	the US has gone to higher power, rather than lower powered
	vehicles, but to a young kid with a new 48 hp diesel Rabbit,
	which seemed to have plenty of power, the idea of slightly
	less power for more efficiency seemed to make sense.

	Earlier this year I bought a 40hp turbine, for about $600K.
	So the cost for cars is still a little out of sight.  We 
	ought to try making our own.  grin.


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