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



On Fri, 2009-01-02 at 15:22 +0000, Howard Winter wrote:
> 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.

FWIW, "shutting off half the cylinders" is done with some engines, and
works. The Dodge Hemi (5.7L) comes in a version that can shut off half
the cylinders when not needed. There are various names for this
technology:

en.wikipedia.org/wiki/Variable_displacement
en.wikipedia.org/wiki/Multi-Displacement_System
en.wikipedia.org/wiki/Variable_Cylinder_Management
http://en.wikipedia.org/wiki/Active_Fuel_Management

The technology DOES work. However, the fuel savings are no where near as
good as some hope for. I've seen numbers between 5 and 20% savings.
Doesn't sound like much, but considering we're talking about some
engines that normally consume 14-16L/100km on the highway, this can be a
substantial savings over a fleet.

A friend of mine has a car with MDS. His usage on the highway goes from
16L/100km to about 13L/100km (MDS has the most effect on light load
driving, i.e. cruising on the highway).  TTYL

<1231001831.9288.101.camel@E2140> 7bit

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