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Thread: : What is involved in making a selectable cylindercombustion engine ?
picon face BY : Howard Winter email (remove spam text)


On Mon, 29 Dec 2008 17:26:34 -0700, Michael Algernon wrote:

> What is involved in making a internal combustion ( gasoline or  
> diesel ) engine with '"n" cylinders operate on "n" divided by two  
> cylinders ?  Does one just turn off the injectors ? Are there other  
> requirements ?  Does one vary the cylinder usage to reduce noise and  
> wear ?  Can a DIY meddler attempt such a project on a 1997 8 cylinder  
> gasoline fueled SUV ?  What are the fuel saving ramifications ?  
> Ideally I would like to drive around on "4" cylinders and switch to  
> "8" cylinders to pull a trailer.

Having (say) a V8 running on 4 cylinders doesn't make economical sense, because you'll need the same energy from the 4 cylinders to achive the performance your
right foot is demanding, and the 4 non-running cylinders will be wasting energy as air pumps - all the suck-squeeze-nobang-blow energy would be wasted.  Added
to which you're pumping slightly warmed air into the exhaust system, which would upset the electronics trying to set the mixture, and may cool down the catalytic
converters to a non-working temperature.  If you had a completely separate exhaust system for each half you could hold open all the exhaust valves on the
non-running cylinders to minimise the pumping loss, but it's still not zero.  But I still thing the only time you'd be saving fuel would be while idling in traffic.

Also, while diesel engines inject direct into the cylinders petrol injection sometimes (usually?  always?) injects into the inlet manifold, so you can't just stop a
particular injector and expect that to exactly starve one cylinder  - remaining fuel vapour in the manifold could result in a very weak mixture entering the cylinder,
with possibly detonating results - or unburnt fuel passing out into the exhaust.

When I was a kid I roughed-out a design for a 4+2 cylinder inline engine, with a clutch between the halves so that one half could be disconnected and stop, and it
had electromagnets to synchronise the two so that it was balanced in the combined configuration, but I'm still not convinced it would have made economic sense.  
The Prius idea of stopping the engine when it's not needed, and having instant-start when it is needed would probably result in better fuel saving than any crafty
engine-size tweaking.


Howard Winter
St.Albans, England

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