Re: GAS-L: Wasted throttle power

[Andrew Heggie <andrew.heggie@dtn.ntl.com>]

On Mon, 19 Feb 2001 11:27:26 -0400, Kevin wrote:

<snipped good explanation>
>
>The conceptually simple way to make an Otto Cycle engine efficient over a
>range of loads is to vary the engine displacement, rather than keeping
>displacement constant, and varying the compression ratio. One way to do this
>is disable/enable cylinders in a multi cylinder engine, while keeping fuel/
>flow to the operating cylinders constant.

This is done on the larger Daimler Benz engines, not easy to implement
in the absence of fuel injection. Also some problems with keeping
internal surfaces hot and pumping losses, you have to cycle some air
otherwise there are problems with sealing I think.
>
>This conceptual solution has ugly practicalities, but thats the only way I
>can see it working.

Some military engines seem to be capable of variable compression
ratio, they are opposed pistons with the combustion taking place
between the pistons and have two crankshafts, however capital cost
must be high as well as friction losses.

Ricardo envisaged a stratified charge engine in 1928 which got around
the problem by having two inlet valves and a separate combustion
space, air+fuel mix entered through one valve, air alone through the
other. The air fuel mixture then burned and heated the air in the rest
of the cylinder.

Another approach used by Honda in their small generators is to vary
the engine speed rather than holding the speed synchronous for a
generator, the generator output then being chopped and reassembled to
a (near) sine wave by solid state electronics.

>  -----Original Message-----
>  From: Reedtb2@cs.com [mailto:Reedtb2@cs.com]
>  Sent: Monday, February 19, 2001 10:32 AM

>  2)  The diesel engine does not throttle the air - only the fuel, so there
>are
>  no "throttle losses" and the diesel burns from lean to very lean and so is
>  very efficient.

Not quite so straight forward, a large high speed diesel may get to
about 40% conversion at its optimum but at part load (e.g. 50%) and
synchronous rpm in both cases conversion will drop severely. Also bear
in mind we normally quote performance in miles per gallon when
comparing petrol and diesel engined vehicles, this favours the diesel
engine as typically petrol and diesel will have similar calorific
values per kg but, being a lighter fraction, petrol will have 10% less
calories per litre.

In fact in its sweet spot (which will be designed to be at about
1500-1800rpm for generators) the engine is working at peak mean
effective pressure, given the same compression ratio the SI engine
will be more efficient than the diesel. 

In fact the cr is more limiting in the SI engine as the Otto cycle
conversion efficiency is asymptotic to about 70% max (in practice I
doubt 50% of this is achieved). Practically engines over 12:1 do not
seem to offer any benefits, even so I think their NOX performance
would be poor.

AJH