General engine questions: Compression ratio and Displacement

[jdbubb@ix.netcom.com (Dan Bubb)]

----- Original Message -----
From: Jonas K. <jkarlsso@metabolex.com>
To: <jdbubb@ix.netcom.com>; Tosha Berk <tberk@mindspring.com>
Cc: <aireq@u.washington.edu>; scirocco list <scirocco-l@scirocco.org>
Sent: Wednesday, June 12, 2002 3:31 PM
Subject: Re: General engine questions: Compression ratio and Displacement


> On 6/12/02 5:28 AM, "Dan Bubb" <jdbubb@ix.netcom.com> wrote:
>
>
> > No! Changing CR will have only a small impact on the volume or mass of
> > fuel and air the engine breathes, so this statement is not correct.
>
> Actually this is wrong. Higher compression will increase the efficiency of
> the motor (as an air pump) at lower RPM. Greater efficiency = more torque.
>
> With a higher CR, the combustion chamber gets "emptied more" at the
exhaust
> stroke, so for the intake stroke there is more suction. Ever play with a
> syringe? Without the needle, of course. For low CR it's like having the
> plunger 1/4" from the end, and putting your finger over the end while
> pulling out the plunger 2". For high CR, have the plunger nearly all the
way
> up, then pull out the plunger 2". There is much more vacuum generated with
> the High CR model.
>
> Similarly, the high CR engine will suck in more air each intake stroke.
And
> also there will be a lower percentage of old exhaust in the chamber.
> However at high RPM, the fill/drain happens so fast that the CR has less
> effect. There is a nearly continuous flow of air pumped by the piston.
>
> This is why the 1.8l High CR 8v motors have so much more pull at low RPM
> than the JH. And also why, even though the JH has the crappy single outlet
> EM, the HP difference is only 10hp (mostly attributable to the larger
intake
> runners and the better EM of the A2 cars).
>
>
> HTH,
> Jonas

The basics of what you are saying is, of course, correct. My statement said
"a small impact". When I say this I'm thinking in terms of going from, say
8.5 to 10 CR, or from 10 to 11. Something we might actually do, as opposed
to going from 8.5 to 14.
The major differences between the JH and RD is 1) CR from 8.5 to 10, 2)
intake with larger longer (better for low end torque) runners, 3) totally
different exhaust manifold and downpipe, and 4) a cam with considerably more
lift and duration (not to mention hydraulic).
Given all the above changes and the fact that the difference in torque
across the 3000 to 5500 rev range is right at 10% I think it's a little
difficult to state emphatically that the increase in volumetric efficiency
due to the change in CR ONLY! is any more than ~2%. To me that is "a small
impact".
But, as I think about this more, the cylinder is not a closed chamber like a
syringe. Certainly, when the piston is at TDC the effect is similar because
the volume is expanding rapidly, but once the piston moves down the bore and
the valve is wide open I would think that the difference of 10 cc in the
size of the combustion chamber in a 450cc cylinder is irrelevant.
Perhaps the biggest impact of increasing CR is that it will generate
stronger vacuum and pressure pulses (similar to a temperary syringe effect
at TDC) in the inlet and exhaust tracts that could be used to increase
volumetric efficiency although I don't really think VW took much advantage
of this.
just my .o2
Dan