|
Dear
Tom
The
practical CR for an Otto Cycle depends on the octane rating of the fuel , among
other things. Engines are typically designed to run with a CR in the range of
say 8, more o=r less, to accommodate readily available fuels without
detonation.
The
12:1 CR would not be intended for a "full charge" of "normal fuel".... it would
knock very badly. However, if the throttle is partially closed, and only a "half
charge" is added, then the CR can be increased to a "nominal 12", but in
reality, it is only a CR of 6, because only "half the normal Fuel/Air mixture"
was added. Temperatures and pressures then stay well below the range where there
is a danger of detonation.
Regards,
Kevin
Chisholm
Dear All:
Having taught thermodynamics and studied the Carnot and Otto cycles, I
was
impressed by the LINEAR increase in efficiency with compression ratio
in the
range CR 6-12. (Above 12 frictional forces reduce the
advantages.)
Therefore, I am puzzled by the SAAB variable compression
ratio approach,
since backing down CR would reduce cycle efficiency.
Unfortunately, real
life is always a little more complex than
any given solution, so we keep
getting in deeper.
Puzzled......
TOM REED
In a
message dated 2/20/01 5:02:40 AM Mountain Standard Time,
A.Weststeijn@epz.nl writes:
On the subject of variable compression of combustion
engines:
Kevin wrote:
> >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.
>
Andrew Heggie
[SMTP:andrew.heggie@dtn.ntl.com]
replied 20 februari 2001 2:35
on
enabling/disabling cylinders:
> 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.
>
Peter Singfield tipped off the
Cadillac engine with enabled/disabled
cylinders.
Now, for a true
example of variable compression check out the new SAAB
engine recently
introduced on the market.
I don't drive one so I can't say whether
the claim of reduced fuel
consumption by 30% is real.
But parameters
of 225 bhp and 305 Nm (225 lbfeet) torque, for a mass
produced 1.6 liter
(98 cu-inches!) engine sold in the regular car market,
are impressive by
any standard!
http://www.saab.com/home/GLOBAL/en/index.xml
(choose subweb "Saab Variable Compression"
QUOTE
SVC - a
unique new engine concept
> Saab is now launching an entirely new
engine concept named SVC, which
> stands for Saab Variable
Compression. Owing to the SVC engine's unique
> design, it offers
performance on a par with units twice its size but with
> the fuel
consumption of a small engine. The SVC engine is a five-cylinder
>
1.6 litre unit producing 225 bhp, and it delivers no less than 305 Nm of
> torque.
>
> What is unique about the SVC engine is
that it features variable
> compression. In other words, the ratio
between the piston's displacement
> volume and the volume of the
combustion chamber is not constant, as it is
> in a conventional
engine. Instead, the SVC registers current needs and
> decides how
much the mixture of fuel and air is to be compressed in the
>
cylinder prior to ignition. The upper part, the mono-head, can be inclined
> up to four degrees to achieve optimum compression, which means that
the
> engine always works at its most efficient level.
>
It is this variable compression in combination with considerable
overboost
and a scaled-down cylinder displacement that makes the SVC
design so strong
and at the same time so fuel-efficient. Generous
overboost means it is
possible to supply more fuel to the engine as and
when needed. This in turn
promotes both greater torque and higher power
output. A smaller cylinder
displacement also means the engine is lighter
and operates with lower
friction, so it uses fuel more efficiently
compared to a conventional
engine. Fuel consumption can be reduced by up
to 30 percent - while
retaining existing performance levels.
UNQUOTE
best regards,
Andries Weststeijn
|
