- One of the common questions we get asked when it comes to engine development
is what compression ratio should I run?
And the compression ratio has a knock on effect to a number of aspects of the engine
operation.
To get a little bit more insight into this we're here with James from Radical Performance
Engines to find out exactly what the compression ratio will affect.
So James for a start we're talking here about your RPE Hayabusa based V8s in particular
and I want to start by asking what compression ratio you're running
in these typically?
Depending on variant, we go from 12.5 to 13.5.
We have gone as high as 14.5 running methanol fuels.
- OK so with the 13.5:1, 12.5:1 region, what fuel are you able to run with that
sort of compression ratio?
- We can run at 98 RON minimum.
- It's probably a good place to also say that these engines are naturally aspirated so the
numbers here would definitely not work for a turbocharged engine.
So on the pump fuels that you're running there, 98 RON, with that sort of
compression ratio, maybe 13:1, are you knock limited?
- Not massively so in these engines, you get to around 32 degrees and you're
into the point of diminishing returns.
You don't really induce knock at that point.
If we really squeezed the compression ratio up to 13:5 you are getting to the point
where it does become knock limited.
- And for those I guess who aren't following that, just to be clear as well,
when I'm talking about knock limited, when we're tuning these engines on a dyno,
we're advancing the timing and we're watching the engine torque and as the
torque climbs we know that we're getting our timing optimised but particularly
on a low grade fuel, we can get to a point where during the combustion process
we get the onset of detonation or knock which is incredibly damaging so that
becomes our limiting factor.
We can't add more timing in fact we need to pull it back to provide a safety margin
so just want to clear that out of the way there.
So with the compression ratios that you've talked about there for pump fuel,
12.5:1 through to 13.5:1, across that range on the RPE V8 what sort of power
difference are you seeing?
- Around 10 to 15 horsepower.
- Where we're starting to talk about the last sort of few percent of engine
performance there, would that sort of compression ratio change?
- Yes.
- Do you also see an effect on the engine's fuel efficiency as you change
the compression ratio?
- Not massively so, we don't look into it a great deal.
- Maybe for an endurance race it might be important but not for the sort of
racing you're typically seeing?
- No.
- Now another term that we talk about when it comes to engine development is squish.
Can you just explain for those who maybe haven't heard that term before,
what exactly squish means.
- Yeah it's the distance between the crown of the piston and the cylinder head.
So obviously we would ideally want as tight a squish as possible so we can just
force all the air/fuel mixture into a central point in the piston so it can be burned
nicely, ensuring complete combustion.
- OK so is there a trade off between squish and your compression ratio?
- Well in an ideal world you'd get the compression ratio as high as possible
and the squish as tight as possible.
But if I had a choice between the two to really maximise, I'd always choose
squish over compression ratio.
- When it comes to increasing the compression ratio, the other thing
we're doing here is basically making the crown of the piston taller or filling up
more of the combustion chamber when the piston is at or about TDC.
So is there a trade off between increasing compression ratio and perhaps ending
up being detrimental to the flame front travel or combustion process
during combustion?
- Yeah if you don't have the sufficient squish to ensure that air/fuel mixture is in the
correct place to burn, you can end up with a very spontaneous burn and
also if you exceed, if you're pushing the compression ratio too high you can
end up with a very fast flame front.
- I think the other thing just to come back to your point about squish,
we're trying to push all of the fuel/air charge into the centre of the combustion
chamber there, is when we do get detonation occurring generally this
will start in the end gasses around the outside of the combustion chamber.
So I guess if we can eliminate those pockets of unburned fuel and air
from there we're reducing the chances of detonation?
- Exactly so if you can squeeze that gap as tight as possible you're forcing all
the air/fuel mixture where there will potentially be a detonation event,
you're removing that potential.
- Look it's been great to get some insight there James,
thanks for your time there.
- Thank you.
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