Compression ratio and volumetric efficiency.

[Stefan Ingi Jonsson]

Does compression ratio affect volumetric efficiency? If so, how?

[dsimila]

Det borde bli tommare i cylindern på slutet av avgastakten med mer kompression.

[Stefan Ingi Jonsson]

Det borde bli tommare i cylindern på slutet av avgastakten med mer kompression.

What about cylinder scavenging with low exhaust/intake pressure ratio?

[Stefan Ingi Jonsson]

"Ratio of Pressures Pe / Pi and Compression Ratio
These quantities work together. The problem is that the presence of residual gas takes up room and lowers the volumetric efficiency.

• See Figure 6-4. Higher compression ratios suffer less loss of volumetric efficiency as the pressure ratio increases.

• Of course as the pressure ratio decreases the lower
compression ratio engines see a higher % increase in
volumetric efficiency."


The text above is from an assignment that professor Jerry Fine PhD(teaches mechanical engineering at Rose-Hulman Institute of Technology) makes for his students, he is using a book that is called "Internal combustion engine fundimentals", this book is the most respected resource on internal combustion engines.

I have not found anyone talking about this phenomenon on any forum which is strange because this is a core fundimental that can be used to extract maximum power from turbo engines.

I wanted to see if anyone in this forum had already learned this, it seems not.

[quattro]

Here's a website that discusses the effects of changing compression ratio - when it's positive for performance and when it's not.
https://dsportmag.com/the-tech/educatio ... 101-part2/
Article Part 1 is less interesting since it's just about measuring.

I came across these articles some years ago and found them very interesting since they explained a phenomena that I had noticed but not fully understood. I have an Audi 80 Quattro 5 cyl 10v rally car. It's turbocharged and the rules dictates a 34 mm restriction in front of the compressor intake. Conventional wisdom says it ought to have a 10 to 12 in CR and and a fairly high duration camshaft. That's pretty close to a basic high performance atmospheric engine set-up.

The problem has always been though, that my engine doesn't like that set-up at all. I have tested a number of different set-ups and it has always preferred around 8,5 in CR and a mild cam with a wide lobe separation angle. My understanding of the reasons behind this was greatly helped by the dsportmag-article referenced above.

And here's my main overall conclusions:
Lower compression ratio lowers the Thermal Efficiency but raises the Volumetric Efficiency. It's because the burn rate slows down which means you need to advance ignition increasing the pressure build up before TDC, plus that more heat escapes into the cooling system. That's partly offset by the increased volumetric capacity (more air in the cylinder because the total cylinder volume including the combustion chamber is greater).
But the overall effect is negative. And a further negative by-effect for a turbo engine is that the cooler exhaust gasses raises the boost threshold level.

But if you at the same time decrease the duration of the cam and widen the lobe separation you can get back more than you lost - by increasing the dynamic compression ratio. It will offset the decrease in geometrical CR and will also substantially increase the amount of air that is captured in the cylinder because the change in valve timing will have an additional important positive effect on volumetric efficiency.

I calculated a total increase in captured volume of air in the region of 12% while keeping the same real (dynamic) CR on my engine when going from a high CR/big cam set-up to a much more conservative set-up. And it definitely shows in the performance numbers.

But this was on a 2-valve turbo engine with a 34 mm restriction. The efficiency improvements will diminish with rising revs. But the mandatory 34 mm restriction's main consequence is a loss of air in the higher rev ranges so the potential negative impact never materializes. On my engines there's a vast improvement in performance in low revs and there's still a small but noticeable improvement at 6000 rpm.

On a 4-valve atmo engine the calculation result will be different.

[quattro]

I wrote And a further negative by-effect for a turbo engine is that the cooler exhaust gasses raises the boost threshold level.
But that statement can be misleading. Higher compression ration makes the exhaust gasses cooler, containing less energy. So lowering the compression ratio helps spooling the turbo earlier.

[Stefan Ingi Jonsson]

Here's a website that discusses the effects of changing compression ratio - when it's positive for performance and when it's not.
https://dsportmag.com/the-tech/educatio ... 101-part2/
Article Part 1 is less interesting since it's just about measuring.

I came across these articles some years ago and found them very interesting since they explained a phenomena that I had noticed but not fully understood. I have an Audi 80 Quattro 5 cyl 10v rally car. It's turbocharged and the rules dictates a 34 mm restriction in front of the compressor intake. Conventional wisdom says it ought to have a 10 to 12 in CR and and a fairly high duration camshaft. That's pretty close to a basic high performance atmospheric engine set-up.

The problem has always been though, that my engine doesn't like that set-up at all. I have tested a number of different set-ups and it has always preferred around 8,5 in CR and a mild cam with a wide lobe separation angle. My understanding of the reasons behind this was greatly helped by the dsportmag-article referenced above.

And here's my main overall conclusions:
Lower compression ratio lowers the Thermal Efficiency but raises the Volumetric Efficiency. It's because the burn rate slows down which means you need to advance ignition increasing the pressure build up before TDC, plus that more heat escapes into the cooling system. That's partly offset by the increased volumetric capacity (more air in the cylinder because the total cylinder volume including the combustion chamber is greater).
But the overall effect is negative. And a further negative by-effect for a turbo engine is that the cooler exhaust gasses raises the boost threshold level.

But if you at the same time decrease the duration of the cam and widen the lobe separation you can get back more than you lost - by increasing the dynamic compression ratio. It will offset the decrease in geometrical CR and will also substantially increase the amount of air that is captured in the cylinder because the change in valve timing will have an additional important positive effect on volumetric efficiency.

I calculated a total increase in captured volume of air in the region of 12% while keeping the same real (dynamic) CR on my engine when going from a high CR/big cam set-up to a much more conservative set-up. And it definitely shows in the performance numbers.

But this was on a 2-valve turbo engine with a 34 mm restriction. The efficiency improvements will diminish with rising revs. But the mandatory 34 mm restriction's main consequence is a loss of air in the higher rev ranges so the potential negative impact never materializes. On my engines there's a vast improvement in performance in low revs and there's still a small but noticeable improvement at 6000 rpm.

On a 4-valve atmo engine the calculation result will be different.

What do you think about the compression ratio of 4 cyl 2 liter drag racing engines(mitsubishi for example) that are running 45 psi of boost and use methanol as fuel, I have seen some of them that have 10 in CR, do they actually get more power with the high CR or is this a blind belief that high CR=more power and they never actually test it?

[quattro]

What do you think about the compression ratio of 4 cyl 2 liter drag racing engines(mitsubishi for example) that are running 45 psi of boost and use methanol as fuel, I have seen some of them that have 10 in CR, do they actually get more power with the high CR or is this a blind belief that high CR=more power and they never actually test it?

High revs, no restriction and meth changes quite a lot. I am sure they have large duration cams as well. All this together means they can - and should - run high compression ratios. These engines are much more extreme than my engine. But I'm sure the have VERY low torque figures in the lower rev ranges. They are optimized for 10 000 rpms or thereabouts.

[Matti johansson]

What duration, loabsep and cr did the 80 like the most?
please do the duration at 1mm if you can.

When you went hi CR, what dur and loabsep was the most successful.

Just to clearify my question, I have a SAAB 9-3 T as rallycar. and the diference between me and Quattro is that i do not have the same "need" for torque, as more for HP. not the number itself but for a longer curve. rev longer.
we both have 34 mm restriction. mine is a 4v, quattros an 2v.

/ Matti

[quattro]

Hi Matti,
The short answer to your question is ”none”. My engine simply doesn’t like a high CR at all. It makes more power and has a wider power band with a cam that has 240 degrees @ 0,5 mm lift / 112 lobe sep / and with 8,5 to 9 in CR, than with a cam that has 265 @ 0,5 lift / 102 lobe sep / and with 10,5 CR.

In my experience the reasons are the following and it all starts with the turbo characteristics. The turbo decides the cam and the cam decides the CR. But the balance can be a bit difficult. This is the long answer to your question.

With an easy-spooling, low backpressure, modern turbo you can easily get a pressure differential between MAP and TIP of 50 kpa at 3-4000 rpm. A long duration cam will help in creating the high MAP since the rising piston will press air back up into the plenum before (and due to) the (late) closing of the intake valve. The high MAP and low backpressure and long cam duration means a lot of fresh, compressed, air will escape out past the exhaust valve during overlap. Not taking part in the combustion. With an inlet restricted turbo that means an important loss of mid range power. The higher the MAP is, and the lower the exhaust back pressure and the bigger the cam - the bigger the loss will be.

To decrease the loss of air you can cut down on the overlap or increase the backpressure. The pressure difference diminish with rising revs. Partly since the 34 mm restriction makes MAP inversely proportional to engine speed and partly because exhaust back pressure naturally increases with rising revs. But the larger the pressure differential, the shorter the overlap (and duration) has to be to stop air from escaping. And the shorter the duration the less efficient the engine will be at higher revs. Its a balance.

Less overlap also means earlier closing of the inlet valve, which in turn means increased dynamic CR. To counter that you need to decrease geometrical CR accordingly. That means bigger compression chambers and potentially higher volumetric effciency. But you need to keep the squish distance tight. Otherwise there will be more exhaust gasses left in the chamber when the exhaust valve has closed – which happens at an earlier point due to the change in cam duration and/or lobe separation angle. So the gas exchange process must be as efficient as possible and this is where 4 valves really outperforms 2 valves. Especially with a short duration cam.

Low exhaust back pressure is simply not a good thing on a restriction engine. Especially on a 2v engine. If you can increase the back pressure to the point where it matches the MAP you will have a much freer choice of cam and thus also CR. But that increase must come from an increased pressure differential across the turbine wheel, not from restrictions in the exhaust system or the exhaust manifold. And the turbo must be "easy-revving" and quick-spooling.

With a low back pressure turbo my engine prefers an even milder cam than 240 degrees.

[Matti johansson]

Thanks for the detailed explaining. i have a lot to test on my car since its very std. even turbo is std aero unit,
and i think i have 5 pairs of cams to test.
just need to find the urge

/ Matti

[quattro]

I don't have much experience in developing 4-valve restrictor engines. Most of what I've learned comes from developing my Audi 10v. I've seen many ups and downs over the years. I've tested compressions ratios ranging from 8 to 12, 5 different camshaft specs, six or seven different turbos and restrictor designs - and 4 different engine management system. In various combinations. But there's still much to learn. Not just in the basic set-up of the engine but also when it comes to programming and tuning.

The basic performance level that is relatively easy to reach should be a bit higher on a 4-valve engine. But as always, the closer you get to the limits the more challenging and difficult it gets.

It would be interesting to see where your engine stands at today, both in terms of "hardware" and performance, plus what level of improvements you would like to see, going forward.

[Matti johansson]

As always dynos are like weight scales. if you pull out a scale at biltema you wigh, 88kg. ica 92kg, coop 78kg. unfortunatly its the same on dynos....
so its a bit hard to compare cars in different dynos.

my dyno is a rollingroad with 4 barrels. and sportdyno sw. and last time i dynoed it, it was maxed out on 288hp i think and restricted to 480nm.
i can get well over 500 nm if i want but on a 2wd its useless.

if i remeber i will post a chart.

its all std internally exept forged pistons. turbo from a saab 9-3 AERO.

/ Matti

[super7]

Hi Matty

A friend drove a Saab 9'3 in a rally about 20 years ago.
It was standard except that a lot of work was done on the exhaust system and the restrictor also thinks that there was a rather large intercooler, then he had access to equipment and knowledge to chip the original Ecu and dyno.
So it was x number of different chip and restrictors before he was satisfied.

[quattro]

my dyno is a rollingroad with 4 barrels. and sportdyno sw. and last time i dynoed it, it was maxed out on 288hp i think and restricted to 480nm.
i can get well over 500 nm if i want but on a 2wd its useless.

I'm well aware of the fact that different dynos will give different results. For a variety of reasons. But over the course of a singel test session they give repeatable results (if you can keep the ambient - or engine intake - air temperature stable) and thus are very good for tweaking and optimizing your current engine set-up.

I would say the numbers you shared are still interesting, especially together with the fact that your engine is very close to standard. It supports the point I'm trying to make, that a 2-valve turbo restrictor engine is much more difficult than a 4-valve to tune to a decent performance level. It's the reason I've found it necessary to test so many different set-ups.

When SBF for a brief period allowed turbocharging on some older, naturally aspirated cars, like the Audi 80 quattro, I asked for permission to change to a 20-valve engine. Even without a turbo. But the answer was always no. So I decided to go with a 5 cylinder 10-valve restrictor turbo. Despite all the work and all the mistakes I've made, it's been very interesting and great fun.