Reduction of compression ratio?

All that has to do with the power train, gearbox, clutch, fuels and lubricants, etc. Generally the mechanical side of Formula One.
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mep
29
Joined: 11 Oct 2003, 15:48
Location: Germany

Reduction of compression ratio?

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I got some heated discussion with some guy if it is reasonable to reduce the compression of a single cylinder 2 tact engine.

He claimed that a big amount of the energy is wasted to build up this compression.
He also said that the amount of fresh air going into the cylinder depends on the compression ratio.

I can't really see why it is reasonable to reduce the compression ratio. As far as I know causes higher compressions ration an increase in efficiency. So usually you want to utilise you fuel and go to the knocking limit or peak pressure.

What do you guys think?

PhillipM
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Joined: 16 May 2011, 15:18
Location: Over the road from Boothy...

Re: reduction of compression ratio?

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You extract a lot more energy on the firing stroke with a higher compression ratio, and also tend to draw more air in on the intake stroke too, generally you'd run the compression as high as you can safetly tolerate!

spacer
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Joined: 01 Nov 2009, 20:51

Re: Reduction of compression ratio?

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Back in my two-stroke days I remember several people claiming that a slightly lower static CR would yield a wider powerband on certain two-stroke setups. Among other obvious differences they generally put lower CRs on manual gearbox engines compared to the dragrace-used CVT engines. This would enhance driveability. Never bothered to check this story back then, but now that you mention it I'm actually quite curious for the thermodynamic reasons behind this, if any.

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mep
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Joined: 11 Oct 2003, 15:48
Location: Germany

Re: Reduction of compression ratio?

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What kind of cyclic process does a 2 stroke engine has?
I assume it is similar to the otto process but which volumes do you use for the calculation? Do you take the ones when the intake/exhaust valve just got closed by the piston?

If it is comparable to the otto process then the efficiency rises with compression ratio albeit it doesn't change much once reached something around 10-12.
So a slightly reduction will probably not harm your efficiency very much and can increase your reliability.

I have no idea how a reduction in compression ratio can improve your powerband and driveability. Maybe it has something to do with ignition.
Are you talking about self igniting engines?

riff_raff
132
Joined: 24 Dec 2004, 10:18

Re: Reduction of compression ratio?

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mep,

You should think in terms of cycle pressures and temperatures. Otto cycle efficiency improves with higher operating cycle pressure ratios, and not static geometric compression ratios.

Loop scavenged 2 strokes, especially those using reed/rotary valves and expansion chambers, can have wide variations in trapping and scavenge efficiencies over their speed range. The trapped charge temperature (T1), pressure (P1) and composition (EGR rate) at the start of compression (ie. exhaust port closure) is what is really important, and not so much the actual geometric CR. The wide variations in T1, P1 and EGR means that the engine may be more prone to detonation under some conditions, and it's conceivable that reducing the geometric CR could allow a more optimum MBT ignition timing overall, and yield better cycle efficiency.

With 2 strokes, over-scavenging the engine can result in increased pumping losses and reduced efficiency. The reversible compression/expansion process within a piston engine is pretty efficient. So there is not much power difference in that regard with changing compression ratios. But with over-scavenging, where intake charge is simply dumped out the exhaust, that does negatively affect efficiency.

riff_raff
"Q: How do you make a small fortune in racing?
A: Start with a large one!"

Robert.Gardner
2
Joined: 28 Jul 2011, 10:14

Re: Reduction of compression ratio?

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There are so many factors to take into account when you alter the static compression ratio.

1) how it it put into effect?
many people have just stuck an extra gasket or 2 under the barrel, but this also changes port timing. While an additional gasket between head and barrel does not.

2) as riff-raff states, its the higher dynamic compression ratio that makes more power, so many parts need to be tuned together to get max gain. Consider your exhaust, whether reed or rotary valve intake, crankcase volume (smaller volume leads to higher pressure differential between crankcase and cylinder, hence faster transfer of mixture), air-fuel ratio throughout engine operating speeds, current driveability, etc.

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Shaddock
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Joined: 07 Nov 2006, 14:39
Location: UK

Re: Reduction of compression ratio?

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I've got 2.5cc model diesel engine that has a T screw on top of the cylinder head to raise or lower compression on the fly. Makes a big difference to it's power output.

willwander
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Joined: 10 Aug 2011, 17:14

Re: Reduction of compression ratio?

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There's a lot to think about on two stroke heads. Rule of thumb is higher compression gives better top end power, but if compression is too high it will get hot, you will lose power and you will get detonation. Somewhere about 10:1 compression ratio is common for 95 RON fuel, if you run higher octane or methanol you can run higher compression.

However, your squish band and squish velocity can be more critical than your compression. This is why just adding another head gasket to reduce compression is generally a very bad thing to do, you will get the worst of both worlds i.e. lower power and more heat/detonation.

Production engines usually come with a poor squish band because they have to take into account manufacturing tolerances and wear to bearings etc.

Generally for two stroke heads squish is more critical for power than compression.

Robert.Gardner
2
Joined: 28 Jul 2011, 10:14

Re: Reduction of compression ratio?

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Its beyond most people to measure their compression ratios with two stroke engines.
Most two strokes advertise their static compression ratio as between 6.5-9:1, and rarely list any dynamic ratio.

Yamaha list 8.9:1 ~10.6:1 for their YZ250, but in practice the dynamic could even get higher than 10.6 at certain rpm's with a competition pipe & mods not including any engine machining.

Many two strokes used to be rated for "Standard" fuel, of 89-92 RON, and would actually give less power with higher octane fuels. Higher octane fuels have a slower, more controlled burn rate, and if the combustion is not quick enough, at very high revs, it may ultimately still be happening in the expansion chamber.

riff_raff
132
Joined: 24 Dec 2004, 10:18

Re: Reduction of compression ratio?

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Robert.Gardner wrote:Its beyond most people to measure their compression ratios with two stroke engines.
Most two strokes advertise their static compression ratio as between 6.5-9:1, and rarely list any dynamic ratio.
Robert.Gardiner,

If you were genuinely interested in studying the effects on performance of static and dynamic CR's in a typical loop-scavenged 2-stroke piston engine, it is not too difficult or expensive.

You would first disassemble the upper cylinder and head, and take measurements of the piston, cylinder and combustion chamber.

Then you would drill and tap a small diameter instrumentation port in the cylinder head. To this port you would attach a check valve, a short length of tubing and a pressure gauge. This would give you peak combustion pressure during operation, over several cycles.

Then you can use a standard automotive compression gauge to measure cranking compression pressure.

The final step would be to download this free single cylinder engine simulator software from Lotus Engineering, and build an analytical model of your engine. Then tweak the model to match your engine's measured parameters. Once you have a validated engine model, you can make changes to any of your engine's parameters and immediately see what effects they have without doing anything further to your actual engine.

http://www.lotuscars.com/engineering/en ... -downloads

Good luck and have fun!
riff_raff
"Q: How do you make a small fortune in racing?
A: Start with a large one!"

Carlos
11
Joined: 02 Sep 2006, 19:43
Location: Canada

Re: Reduction of compression ratio?

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You might enjoy:
Two Stroke Tuner's Handbook by Gordon Jennings
http://2stroker.net/jennings/

A free download from his memorial archive, and it's a good thing to make a small donation via PayPal to keep the site active.

Robert.Gardner
2
Joined: 28 Jul 2011, 10:14

Re: Reduction of compression ratio?

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riff_raff wrote:
Robert.Gardner wrote:Its beyond most people to measure their compression ratios with two stroke engines.
Most two strokes advertise their static compression ratio as between 6.5-9:1, and rarely list any dynamic ratio.
Robert.Gardiner,

If you were genuinely interested in studying the effects on performance of static and dynamic CR's in a typical loop-scavenged 2-stroke piston engine, it is not too difficult or expensive.

You would first disassemble the upper cylinder and head, and take measurements of the piston, cylinder and combustion chamber.

Then you would drill and tap a small diameter instrumentation port in the cylinder head. To this port you would attach a check valve, a short length of tubing and a pressure gauge. This would give you peak combustion pressure during operation, over several cycles.

Then you can use a standard automotive compression gauge to measure cranking compression pressure.

The final step would be to download this free single cylinder engine simulator software from Lotus Engineering, and build an analytical model of your engine. Then tweak the model to match your engine's measured parameters. Once you have a validated engine model, you can make changes to any of your engine's parameters and immediately see what effects they have without doing anything further to your actual engine.

http://www.lotuscars.com/engineering/en ... -downloads

Good luck and have fun!
riff_raff
To try to measure the true Dynamic Compression ratio, we need to operate the engine throughout its intended operational rev range, with its full exhaust and airbox. Any variable exhaust port system would also be taken into account.

So any measurements of the pressure in the combustion chamber would need to be made at a precise time prior to ignition (once ignition occurs, the pressure will rise quickly). We then need a piezoelectric transducer to monitor the pressure, and something like this
http://www.sokken.co.jp/en/pressure/201011/entry8.html
to calibrate it.

In our measurements we find that the heat within our cylinder has already affected the pressures of our incoming charge, and we can not measure at TDC; so must either extrapolate our measurements, to cater for TDC, or possibly for the point of ignition.

And thanks for providing the links, Carlos & riff_raff.

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