compression ratio

[gttouring]

I read and read that the ideal compression ratio in a car with a turbo charger is 8.0:1
now with a natural motor on regular gasoline the best possible would be 13.75:1 (make sure it is full of octane heh, 110 octane rating a little lead anyone)
so normal street cars run 9.5 to about 11.5 (toyota matrix):1.
Turbos increase cylinder pressures so compression is best kept low, so how is 9.5 or 9:1 as a turbo compression?
can it be done really well? (I am currently making a track car and well, th lotus is on hold i might as well build this thing)
and what kind of compression does Audi and the Champcars run?

[Speck]

AKAIK, the FSI engined AUDI's and the BENTLEY runs at about or over 12.0:1 comp. ratio. I have heard that the Toyota GTOne ran with a 9.0:1 comp. ratio, but can't really remember where i got this info. The thing is, with present day LeMans rules for prototypes, the amount of boost allowed to run for each car/engine is limited, down to 0.5 bars for a 4 litre engine (1.5 bars absolute pressure), so the engine people would either have to raise comp. ratio, or raise the RPM to try to get some more power.

[graham bahr]

in both turbo and naturally apasarated engines the compression ratio that can be run is directly linked to the volumetric efficency of an engine, i.e how well you can fill the cylinder, generally the highest you can run a naturall asparated engine with good breathing on normal super unleaded is 12.1, however introduce a restriction in the intake and the cylinder filling goes down so the compression ratio can go up, i know one engine bulder who runs a compression of 15.1 on normal fuel on his 16v vauxhall lump, but it does have to breath through a single 26mm restrictor.
turbos are the same only the the other way round, more boost/ cylinder filling + lower compression.
the other things that will effect the compression you can run are the engines natural resistance to detonation, get the chamber shape and sqwish clearances right and it will burn better and allow a bit more compression, charge temp has a massive part to play, the cooler the charge the more compression, but make the intercooler too big and your just get lag, needless to say the ignition/fuel management must be right!!

you do not say what sort of engine you want to turbo, what are others doing with it? if you are starting from scratch 8.1 seems a reasonable starting place.

boostpressure can be missleading you can has a massive boost pressure but the engine actually sees very little of it as its due to poor breathing because the inlet cam timing/lift is too mild.

dont forget the drivability factor, generally higher boost requires bigger turbo's which means more lag and drivability suffers, lap times will generally be faster if you have instant grunt out of a corner, rather than a big lag followed by a big and rather uncontrollable surge of power.

my 2.0 naturally asp car will often out qualify turbo cars with twice the power due to a much better and predictable power delivery.

consider relieability too, how many turbo cars do we read about now that have a claimed 500+ brake, but when they are tested/run have the boost turned down so they have much less, for relieability sake? loads, inother words, if you cant crank the boost up without blowing the engine you haven't really got that much power have you? you've probably just spent a lot more money to get a less flexable power plant.

[norman-normal]

Cylinder pressure at ignition is the only thing that matters. Comp ratio is constant but cyl pressure is not; it is varieable dependant on a number of things (above listed). A major consideration is altitude; unless all your racing is in flat-lands, it can be a problem.

[avsfan733]

comp ratio is not constant, it is effected by the overlap and openings of the valves as the compression stroke begins

[turboteener]

Static compression is constant in an assembled motor. Dynamic compression is influenced by factors such as rod length, camshaft design, static compression, airflow through ports, engine etc.

Let me be devil's advocate for a minute:

Who says an engine can't run 10.1, 11.1 ,12.1 etc compression in a boosted motor? (and not destroy itself) Wasn't there a time when top fuel cars weren't supposed to go faster than X speed because some formulas said they couldn't? If everyone else is running 8.1, what makes that the best thing to run?

[graham bahr]

i didn't say 8.1 was the best thing to run, i said it it would seem a reasonable starting point, the best compression for any engine/set up can only be determind with development/testing and quite possably a few blown engines

[Chucky]

Peak cylinder pressure is what it is all about. You can run very high static comp ratios if you are able to control the ignition timing very precisely, something which was near impossible until the advent of digital electronics. Although some sprintcar engine builders were running as high as 18:1 on methanol and magnetos. State of the art F3 engines run between 15:1 and 17:1 but this is to counteract the air restrictor. This is probably the best example of static vs. dynamic comp. As the engine accelerates it goes from having peak cylinder filling capacity at 3000-4000rpm to it tapering off to very little at 7000rpm. Hence at max. torque (approx. 5800rpm) it's probably only a 11:1 engine.

A turbo is almost the reverse as th have bad cyl. filling at low boost/low rpm and then are almost hyperventilating at high boost/high rpm.

The problem with very high comp ratios is that your pumping losses start to negate any gain in having big comp. I've only heard rumours of what the current F1 engines use but the numbers have been as high as 16:1.

[gttouring]

well the motor would be a nissan 2.4 litre
i am using this block and crank because- a)it is Iron, and can stand a bit more than aluminium, and has Oil squirters up into the bottom of the pistons, so good natural lubing and cooler pistons, and the nice big torque on the bigger motor than the 2.0 nissan. it revs stock about 500 rpm less, but a computer and balancing the bottom end- and the springs and valves being high rpm types (double springs-steel, Ti valves and retainers)
the compression ration in the motor is rated at 9.5:1 a factory ring set up, which i don't see why it should be lower with proper Rods and Pistons taking the beating.
this should over come the lower revving nature due to the motor's stroke length. so more power and torque can be had.
a rev limit at about 8.5-9K is all I intend. i have the parts layed out and am pondering a dry sump as well...
the car was used as a 'compact touring class' and it is too old to be eligible so it should go to Compact superclass (no real restrictions, just no V8's)

[RWC]

"the motor is rated at 9.5:1 ...which i don't see why it should be lower with proper Rods and Pistons taking the beating"
It's abit more serious than a slight increase in pressure.
The whole thing about compression ratios is to go as high as possible without getting detonation or pre-ignition.These things are serious -they make pressures absolutally skyrocket and can ruin an engine in seconds!

Grahams advice of 8:1 + as a starting point is sensible but idealy try to get advice from someone experienced with that particular motor
good luck with it!

[MA2]

Originally posted by avsfan733
comp ratio is not constant, it is effected by the overlap and openings of the valves as the compression stroke begins

Ummm... Compression RATIO is always constant (except is a new engine from Saab), as it is merely the ratio of the total cylinder volume at BDC vs TDC. Those volumes are fixed.

Peak cylinder pressure, as was pointed out, IS affected by cam lift, duration, volumetric efficiency, etc.

[PeterMorley]

Originally posted by gttouring
I read and read that the ideal compression ratio in a car with a turbo charger is 8.0:1
now with a natural motor on regular gasoline the best possible would be 13.75:1 (make sure it is full of octane heh, 110 octane rating a little lead anyone)
so normal street cars run 9.5 to about 11.5 (toyota matrix):1.

Surely it depends on what boost pressure you are running?
The more boost the lower the CR?

There is a theoretical ideal compression ratio for a particular fuel (I thought it was around 12:1 for petrol) - of course real world losses would vary this slightly, but it is a reasonable starting point.

With your boosted engine you would want to end up at the same figure, which means the static compression ratio should be such that your final compression ratio is 12:1 (or whatever figure you want).

p.s. Is there some confusion about air:fuel ratios and pressure (I thought ideal air:fuel for petrol was around 13.5:1).

[HiRich]

I did my degree dissertation in this, though some 20 years ago. So apologies now if someone spots an error, and for repeating others in a more techy way.

There is no ideal compression ratio for all engines. Volumetric efficiency in a spark ignition engine rises with increased compression. However pumping losses (sucking air in, and the energy required to compress the mixture) also rise, So for one particular engine there will be an optimum level balancing extra energy input and losses.

The controlling factor is pre-ignition. If you can eliminate causes, you can raise the compression ratio, and improve volumetric efficiency. As pre-ignition (in all its forms) has two causes, these have to be addressed:
- A hotspot (e.g. an exposed thread on the spark plug, hot enough to act as a spark. More difficult to predict, a particle of carbon deposit can do the same)
- As the mixture is compressed (first by the piston rising, secondly by the flame front compressing the mixture ahead of it), the pressure and temperature rise. The fuel breaks down into more volatile products which can spontaneously combust.
Solutions are:
- Eliminate the hotspots, particularly sharp edges which can collect heat.
- Increase the speed of combustion. This usually involves increasing turbulence in the mixture (in a controlled way) - Top engineers have fancy software to model this.
- Altering the fuel type to one that doesn't breakdown the as quickly.
- Altering the mixture. You can do this overall, or achieve different mixtures in different parts of the cylinder (known as stratified charge - there is one ideal, richer mixture required to initiate a flame, and a leaner mixture required to maintain the flame).
- Alter the timing (advancing it), so there is less time at the extreme pressures and temperatures (and lower peaks).
- Direct fuel injection into the cylinder: also reduces the time available for the fuel compounds to degrade, and has other benefits.

Getting back on point, if you turbo (or supercharge) an n/a motor, the air entering the cylinder is already compressed and heated. Therefore the amount you can compress it in the cylider has to be reduced (as explained by others).
- Higher pressure boost means the cylinder compression has to be reduced (ie lower compression ratio)
- Cooling the charge (an intercooler) allows you to regain some of the lost compression ratio.

So gttouring, there are no hard & fast rules. You need a lot of experience, or a healthy supply of development engines to make confident predictions. And, as already mentioned, a weather eye on pistons, con-rods, big & little end bearings and other drive-load bearing elements.

[avsfan733]

Originally posted by MA2
Ummm... Compression RATIO is always constant (except is a new engine from Saab), as it is merely the ratio of the total cylinder volume at BDC vs TDC. Those volumes are fixed.

Peak cylinder pressure, as was pointed out, IS affected by cam lift, duration, volumetric efficiency, etc.

sorta true...it does vary because of the valve timing overlapping with the start of the compression stroke....so depending on timing, you start compressing a 100cc charge, but the valve is open until the volume is down to 90 cc. If you were using 10:1 comrpession ratio, the effective ratio would be 9:1 instead because of the loss comparedto your calculations when you choose the piston

[MA2]

Sorry, but the things you mentioned affect only the efficiency, not the actual ratio. As stated before, the ratio quoted is always the ratio of the volumes at TDC vs BDC.

[Chucky]

Dynamic compression ratio varies all over the place as it is affected by the cylinders ability to fill itself. This is the most relevant (and hardest to measure) number you need.

Static compression ratio is what you measure on the bench in the workshop and is just a refernece point.

[avsfan733]

exactly, thank you for a better explanation chucky

[baker]

compression ratio= swept volume + combustion chamber volume / combustion chamber volume

[tristancliffe]

Originally Posted by baker

Static compression ratio= swept volume + combustion chamber volume / combustion chamber volume. The Dynamic CR varies.

Fixed

[GORDON STREETER]

Originally Posted by baker

compression ratio= swept volume + combustion chamber volume / combustion chamber volume

Right OK

[Al Weyman]

and.....

[GORDON STREETER]

Anyone else ?

[Chris Y]

It's the sum of the square of the height of two giraffes, multiplied by the visible percentage of the moon in the night sky, divided by the swept volume of a pig's stomach.

Times two.

[Al Weyman]

...will give you the answer to that age old question, how many apples in a pound of pears.

Sorry Baker and welcome to the site just a bit of old boy banter. :-)

[phoenix]

Whether I am right or wrong, what I use for dynamic CR is the VE of the engine at a given speed and the compression ratio from the point the inlet valve closes until TDC rather than from BDC to TDC plus the pressure ratio (i.e. not the boost level but the increase in charge density over atmospheric, which allows for the increase in charge temperature). This allows me to compute the compression pressure and predict the correct time to ignite it.

It is only 'rule of thumb' but is useful when building an engine from scratch and mapping it from scratch too.

Our 8 valve engine has peak VE of 102% at 4500 rpm (peak torque too, of course). Our inlet cam is on it's seat 52 degrees after BDC. Our stroke is 90mm and the rod length is 145mm. Lets say we use a static/geometric CR of 8.1:1 Because the piston is some way up the bore before the inlet valve closes the gas trapped as that point will be compressed from a volume of 6.94 into a volume of 1 - i.e. 6.94:1 CR.

At one atmosphere (i.e. normally aspirated) peak compression pressure (i.e. at TDC) would be 1 * 102% * 6.94^1.28 bar which is 12.24 bar.

At 1.5 bar boost the density ratio is 2.262:1, so the peak compression pressure would be 27.7 bar. This is the equivalent pressure that a 13.4:1 CR NA engine would produce.

With this in mind, we built it at 7.5:1 CR giving a 'dynamic' CR of 6.43:1.

Same calculation: 2.262 * 102% *6.43^1.28= 25.12 bar - the equivalent of about 12.4:1 NA.

Here is a useful little formula (adapted from one included by John D Humphries book on super and turbo charging)

crB = 1/(((p1/p2)*.73)/crNA)

where:

crB is the boosted compression ratio you are looking for
p1 is the boosted pressure (I use density ratio)
p2 is the normal pressure - 1 bar in our case
crNA is compression ratio that the engine will run normally aspirated without detonation.

This obviously assumes the same cam timing as the original engine and the same volumetric efficieny too.

using 12.5 as the safe maximum NA cr and 2.262 density ratio, this little formula gives 7.57:1 as the safe CR for a boosted engine. At 12.4:1 NA it allows for a 7.51:1 boosted CR.

Using the 0.5 bar boost on a LMP type car, pressure ratio is likely to be about 1.435:1. Using the above formula you can see that the CR would only need to be reduced from 12.5:1 to 11.93:1 to achieve a safe CR when boosted (If 12.5:1 was a safe CR to start with)

Just an addendum. Porsche seem to build all their NA road engines with a geometric CR of 11.3:1. For a turbo 4 valve per cylinder engine running on pump fuel, I would - indeed will - use this as the safe maximum NA CR when building our 16 valve turbo engine. Becasue the VE of a four cylinder engine is so much higher than a two valve per cylinder engine, the dynamic CR obviously increases quite a lot. It maye be a bit conservative - but it shouldn't melt pistons! The gain in overall engine efficiency between 10:1 and 15:1 is about 10% however, so higher CRs are well worth exploring - if you can afford the development costs!