Re: GAS-L: Wasted throttle power

["Graeme A. Bentink" <gbgpss@iinet.net.au>]

Title: Re: GAS-L: Wasted throttle power

Dear Kevin/Tom/Phil,

Modern engines are able to run a much higher CR due to combustion chamber design (including the piston crown shape and ring height since these are part of the combustion chamber) and inlet tract design. Camshaft profiles also affect combustion chamber filling.

Typical 96 octane unleaded petrol can run at 12.5:1 without detonation as with the current Yamaha YZ426 single cylinder Dirt Bike. I have ridden this bike and there is no hint of detonation. 

I have seen a turbocharged 4 liter 6cyl car (Ford) running 8Psi boost and standard in every way except for fueling - even the ignition and cam timing is standard. The CR is higher that 9.5:1. Fuel is 96 RON.

All the best, Graeme

> 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
> > -----Original Message-----
> > From: Reedtb2@cs.com [mailto:Reedtb2@cs.com]
> > Sent: Tuesday, February 20, 2001 7:02 PM
> > To: gasification@crest.org
> > Cc: Philreed@aol.com
> > Subject: Re: GAS-L: Wasted throttle power
> 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