a small question about mapping
i was mapping a porsche 944 3.0T @ 1 bar
i got the ignition upto 21 degrees right after torque drops (19 deg around max torque) and AF was around 11.3
egt measured in 5th gear near limiter went upto 950deg and efter releasing throttle it climbed to 1000 (because of no fuel injected so no cooling? )
but my problem is when we raise boost lets say to 1,5bar
I assume ignition will be around 15 degrees there and probably higher egt in 5th gear but how to prevent that? Better air cooling and little flooding with fuel? (10.x af? )
if a car like taht would need to go flat out for more than 1 mile distance then how to make it safe?
thanks!
Ahh the eternal compromise of VMax testing....
But we're getting ahead of ourselves. Lets try and break the question down slightly. First, what is a safe target EGT temp?. This does vary with the engine and turbo slightly. There are two issues here - the cylinder head and the turbine wheel.
Modern turbo's are generally safe to around 950C continuous. That is gas temperature, in the centre of the flow, measured directly before the turbine entry. Some higher spec turbos (i.e. those with inconel wheels) can survive up to 1050C continuous without damage, assuming you have the right cooling flows to the core.
Cylinder heads are more tricky. We'll forget about single exhaust valve engines, as these are now considered 'old'. For twin exhaust valve heads you need to consider if the head was originally designed for a turbo engine or an NA.
Typically, NA heads cram as much valve area in as possible, and as a result the bridge between the valves is quite narrow. We typically map NA engines to 850C continuous in the ports.
Turbo heads however, tend to use smaller valves, and space the exhaust valves further apart. They may also have a better cooling passage cast into the head to help remove the ineveitable heat.
So, assuming you haven't 'improved' your turbo head by fitting larger valves, and its reasonably modern (say from the mid 90's onwards) so made from sensible materials, you can map to around 980C continuous in the ports, which conveniently equates to around 950 pre-turbine (depending on your manifold design and lengths).
So in summary, old, 2 valve or originally NA heads, go for about 820C continuous pre turbine temp. For modern 4 valve heads, around 950C continuous pre turbine should be OK.
Next, how do we control to our target temp? Again, it depends on the engine.
The typical way of keeping the EGT low is by adding more fuel. However, you need to understand the background to that decision.
You get high EGTs through a combination of reasons. More heat generated in combustion than the combustion chamber can remove, cam timing ( early exhaust valve opening means combustion has not finished before gasses are expelled), high compression ratio, leading to a requirement for ignition being retarded from MBT (see the mapping guides) to avoid knock again meaning combustion hasn't finished before the valve opens. There are also secondary effects such as high charge or coolant tempertures requireing retarded ignition to avoid the increased chance of knock.
So back to the question - how do we control the EGT? Well, it depends on your engine application.
If all you want is peak power, and nothing else matters, then you need to extract the maximum power from every gram of air you pump into the engine. In this case, you would lower the compression ratio until you are running at MBT at peak power. I'll skip over the issue of squish here as its a complex subject, but when lowering the C/R it shouldn't be neglected or you may just trade off one advantage for another. Anyway, In this case the lower C/R will allow you to advance the ignition, which will lower the EGT as the gas temperture will be converted to piston work without knock occurring. The trade off is appalling fuel economy and low torque at part load conditions - not good for a road car, or any race car that need flexibility.
Alternatively, you can still use the 'advance the ignition' trick by using knock suppression methods - water, water/methonal injection, or high octane race fuels will do this, allowing you to keep a more sensible compression ratio for part load, but also run closer to MBT at peak power. The down side is that 120 RON race fuel is expensive, and for many racing classes illegal, and water injection is complicated, needs refilling, adds weight and if it fails to work correctly, and you have no safety system, it can lead to you melting your engine.
Cam timing will have an influence, but typically you won't move the cams to lower EGT's - their position is a compromise for peak power vs low speed response, and will be set with tah in mind
And so, that brings you around to over fuelling. Most gasoline engines make peak power at a lambda of around 0.86. so anything lower than this is termed 'over fuelling'. Thats doesn't necessarily means its wrong (all the production turbo cars I've worked on overfuel significantly at peak poswer), but it simply highlights that the additional fuel is no longer being used to directly make power, but is being used for another purpose - in this case, the additional fuel, which does not get burned in the chamber is soaking up heat, which is whay it both improves the knock limit slightly (it lowers the combustion temp, reducing the chance of knock) and the EGT - partly due to the lower combustion temp, partly because you can then add a bit of ignition advance back in and partly because it keeps soaking up heat from the gas after the valves open and it leaves the chamber.
But, you can only go so far with fuel. Eventually, you add so much fuel to reduce the temperature that the mixture it too rich to reliably combust - this is the rich misfire limit, and is somewhere around the lambda 0.7 to 0.65 region, again depending on the engine and speed load condition.
So that leads to the final option - turn down the boost. Sometimes, when you have made your other compromises, you just can't make as much power as you would like, and thats life.......
One pint on overrun increase in EGT - if there is no fuel then intheory there is no heat being generated. The possible exceptions to this are in the transient region wher you switch from heavy over fuelling to fuel cut, the rush of fresh oxygen which doesn't get used in combustion combines with the residuel excess fuel left in the system, which then ignites on the glowing turbine, sometimes assisted by a retarded ignition that may be put into the minimum load line (as a way of reducing torque at the transition to fuel cut to avoid shunt). This should only be a brief spike though - only lasting as long as the residual fuel takes to burn off. As long as you don't have a catalyst fitted still (which might melt of you keep subjecting it to 1050 / 11 00C), the system can usually take it. If it takes longer then you need to find out why - you only get heat when you have fuel and air, which you shouldn't get if you are in fuel cutoff.......
So, finally to try and be specific for you case, I'm guessing 950C continuous pre turbine is on the high side - its a reasonably old engine, so I'd have some concerns over the cylinder heads (though it may be fine - I don't have any direct experience of that engine). I'd go more towards 900 to start with if thats possible.
Next, you need to make sure you are running at the most advanced ignition timing you can safely run. I don't know about your specific engine - at peak power this may be back at MBT, or you may be knock limited. Only some proper rolling road time will allow you to truly optimise your ignition timing. But you want to do this anyway as there is probably more power to be had there!
Finally you can adjust the fuel until you achieve a sensible EGT. Typically this is done during the ignition mapping as there is the tradeoff between fuelling and knock that I mentioned above. As a general rule lambda 0.86 it the target for peak power, 0.80 is about as rich as you want to go, though I have worked on a production project that had to go as far as lambda 0.75 to control EGT - though by this point piston and bore washing can start to become a problem, and there will begin to be black smoke from the exhaust, so its not recommended unless you really have no other choice.
Hope that helps and good luck (and excuse the spelling mistakes)
Cliff
