Dynamic compression, turbo's and cam's

I've looked into this a bit, and have found numerous sources saying that a mild cam is better for turbocharging because of the situation you describe - a long cam overlap will mean some of the inlet charge being blown back out the exhaust.
Someone may be able to give you a fuller explanation / shoot me down in flames, but that's how I understand it.

Its all about overlap. You are leaking charge if the cam has a lot of overlap and thats defeating the object of forced induction. Stock cams designed to minimise emissions are always claimed to work well with moderate turbo applications, but theres nothing to stop you having a custom ground cam with minimal overlap and an extended exhaust opening to minimise pumping losses. Is this an OHC or OHV engine? Would high ratio rockers be an option if its OHV?

I know very little about turbos I'm afraid.

OK I see what you are saying now. A lot of people say IVO/IVC are the two most significant timing events when tuning an engine and that applies as much to N/A as forced indution engines. I know from my own projects that phasing a cam to open 3* earlier or later can make a tremendous amount of difference to how the motor runs, and much of this is due to the relative positins of the piston and the intake valve. Your motor being a turbo does not rely on atmospheric pressure or scavenging effect from the exhaust in order to maximise cylinder fill so if anything surely you have a bit more flexibility to maximise you effective stroke - although this will probably involve a custom ground cam unless you have a common application. Also you're going to want better timing adjustment than a 3 way sproket gives you, I presume you can get some kind of adjustable vernier pulley for your engine? I'd also also have supposed that fully mechanical valve gear would help with the precission here, although you can use a solid tapet to set up hydraulic actuated valves for purposes of setting up but you can't guarantee that a hydraulic is going to operate exactly like that in use...

I have just read all of that - and am now off for a lie down!

You want to give me the theory on Supercharging then? I have a Suzuki Alto - the crank pulley has a spare drive wheel attached for the non-existant power steering and I have a Merc blower........ help me prevent a complete detonation please! ;D

Actually - I'll leave that one for another thread. Got the Triooomphs to slam first......

Cheers for the replies.

BlownImp, I understand what you're saying and I agree, but I've read in a few places that it may be better to lower compression than to retard ignition when trying to avoid detonation. This is because the location of peak pressure in the cylinder needs to be around 14 degrees ATDC to do maximum work on the piston.

The following passages are taken from here if you want to read it all:

www.germanmotorcars.com/Detonation.htm


"There is another factor that engineers look for to quantify combustion. It is called "location of peak pressure (LPP)." It is measured by an in-cylinder pressure transducer. Ideally, the LPP should occur at 14 degrees after top dead center. Depending on the chamber design and the burn rate, if one would initiate the spark at its optimum timing (20 degrees BTDC, for example) the burn would progress through the chamber and reach LPP, or peak pressure at 14 degrees after top dead center. LPP is a mechanical factor just as an engine is a mechanical device. The piston can only go up and down so fast. If you peak the pressure too soon or too late in the cycle, you won't have optimum work. Therefore, LPP is always 14 degrees ATDC for any engine."

"Production engines are optimized for the type or grade of fuel that the marketplace desires or offers. Engine designers use the term called MBT ( Minimum spark for Best Torque) for efficiency and maximum power; it is desirable to operate at MBT at all times. For example, let's pick a specific engine operating point, 4000 RPM, what, 98 kPa MAP. At that operating point with the engine on the dynamometer and using non-knocking fuel, we adjust the spark advance. There is going to be a point where the power is the greatest. Less spark than that, the power falls off, more spark advance than that, you don't get any additional power.

Now our engine was initially designed for premium fuel and was calibrated for 20 degrees of spark advance. Suppose we put regular fuel in the engine and it spark knocks at 20 degrees? We back off the timing down to 10 degrees to get the detonation to stop. It doesn't detonate any more, but with 10 degrees of spark retard, the engine is not optimized anymore. The engine now suffers about a 5-6 percent loss in torque output. That's an unacceptable situation. To optimize for regular fuel engine designers will lower the compression ratio to allow an increase in the spark advance to MBT. The result, typically, is only a 1-2 percent torque loss by lowering the compression. This is a better tradeoff. Engine test data determines how much compression an engine can have and run at the optimum spark advance."


Now the decision whether to lower compression based on the above statements is dependant on the combustion chamber burn rate of the engine in question (Hillman Avenger all iron OHV in this case). Since I haven't run the engine yet I don't know whether it will suffer detonation or not with the proposed boost levels (12psi at first). However, it has a biscuit shaped combustion chamber which probably doesn't have a very fast burn rate. This means that it would need more ignition advance to burn all the fuel in the chamber, increasing the risk of detonation as the burning fuel has more time to heat up and compress, therefore a lower compression ratio may be required.

Phew! I got a bit carried away there. Any more views welcome.

Cheers

Increased burn speed due to better mixture distribution caused by turbcharger turbulence.
Decreased burn speed due to increased mixture density.
Uneven distribution due to single carb multi cylinder setup.
A carb setup that wont be fulling 100% at all times.
All iron engine thats prone to detonation due to low thernal conductivity.

All of the above affect burn speed, and so your wiebe function changes and so does the peak pressure in the cylinder. What really matters is the area under the graph of your cylinder pressure Vs crank angle, this is what dictates the actual torque output and the amount of energy transferred to the piston. In a turbo engine it dosent matter if the peak pressure moves around a bit due to the fact that the overall amount of pressure normally climbs a but slower and drops slower, making for a 'fatter' pressure curve.

The thing to look at is what manufacturers are doing in WRC, most are running high boost high compression ratio engines. These will run perfect timing at lower RPM to make the engine tractable off boost, then as it comes on boost they pull timing out to prevent detonation, then as the restrictor comes into play the timing is put back in again to make more power in the high RPM areas.

What kind of biscuits are we talking here?

End of the day its not a full on WRC engine, i suggest getting it running and see how it performs. Moving your peak pressure either side of 14 degrees atdc is going to be impossible to calibrate or detect imo (unless you have a sparkplug piezo pressure transducer). You will however be able to detect detonation and remove it by pulling timing out.

if it runs tune it, if it dosent run get it running

J

I have no idea what you're all on about, but I love it and I'm feeling proud to share a forum with you! ;D

Holy hell! - Just back from a lie down and find that lot!

Do we need a visa to visit your planet???

I actually got a lot of it - that's the scary part!

Other than the fact this sounds like a wicked project and I look forward to seeing the results, nothing much to add.

Other than really I'd be looking at a decent cam optimised for your setup but I think you're right to get it running first and then look at enhancements.