Discussion and questions related to the course Understanding AFR
From your experience you have suggested to use a lambda around 0.68 on a turbocharger application. But what about cooling ability of methanol which is much better than pump gas or E85. Heat of vaporization for methanol -1147 kg/kj, gasoline -307, ethanol -873. It means that it does not need to run much richer relatively than either gasoline or E85 in oder to control combustion temperature.
Detonation with alcohol fuel is really not a problem, but pre-ignition is, or could be if the mixture is on the weak side. It burns slowly and can still be so doing when the exhaust valve has opened which then becomes overheated. This in turn ignites the next charge before the correct time, the whole process becoming a chain reaction causing even more rise in temperature and so it goes on until the piston holes and other damage then follows.
The Solution for using the full potential of alcohol fuel - Correct lambda about 0.8 to 0.83, cold spark plug with a normal gap and powerful ignition coils
The burning speed is also dependent on the air-fuel-ratio. At about 0.85 to 0.9 lambda the mixture burns fastest. A leaner mixture than that burns slower. A richer mixture also burns slower. That's why the maximum power mixture is at the fastest burn speed
Faster flame travel = slower timing.
Slower timing = less negative power.
Less negative power = more power on the flywheel
Slower ignition timing makes higher exhaust gas temperatures. There will be a balance between raising the boost and making the timing slower. At some point you will be better off to have slightly lower boost with slightly faster timing.
What do you think about this strategy of tuning engine on an alcohol fuel?
Hey Vladimir and thanks for your questions. I understand the logic you've applied to methanol fuel however I've been involved in tuning a wide range of high boost methanol fuelled engines over my career and I can assure you that tuning at 0.80-0.83 in most instances wouldn't end well.
Methanol is an unusual fuel in that if we looked at power vs lambda, we find that there is very little drop off in power as we move rich. In fact the rich limit of methanol fuel is often enforced by the ignition system failing to ignite the fuel/air charge. This is unlike petrol where we will see a sharp drop off in power once we move richer than perhaps 0.75 lambda or thereabouts. On the other hand methanol is not very forgiving of a lean mixture. Some of this can be explained by the chemical formulation of petrol compared to methanol - Gasoline is made up of complex mixtures of hydrocarbons and each of these hydrocarbon fractions burns at a different rate. In comparison methanol is made up of simple mono mixtures that burn at the same rate. This means that with methanol fuel the onset of abnormal combustion happens very quickly and is why we don't want to be tuning this fuel anywhere near the edge.
One of the characteristics of methanol you mention is in fact a reason why it's beneficial to use a rich AFR - Latent heat of evaporation. As you point out, methanol does a great job of removing heat from the intake air and combustion charge thanks to its latent heat of evaporation and this is one reason why many drag racers choose to eliminate the intercooler on methanol fuel. Of course in this instance we gain benefit from this cooling effect by running a richer AFR as we have more fuel available to pull heat from the intake air.
I'll discuss preignition briefly as this is a concern on alcohol fuels, particularly when combined with high loads. Preignition occurs when the autoignition point of the fuel is reduced far enough that it can be auto ignited at relatively low temperatures by a hot surface in the combustion chamber - It's most likely that this hot surface however will be the spark plug and not the exhaust valves. One of the factors that can promote preignition is oil dilution of the fuel and in this situation alcohol fuel seems to be effected more by oil dilution than petrol - At the same time due to the richer mixtures we tend to run, there is more fuel in the cylinder and the chances of wetting of the cylinder wall and hence dilution via lubricating oil is more likely.
Hi Andre. Do your comments apply to running straight Ethanol (E98) also? A friend was tuning his 4G63 last night on E98. AFR was at between 0.8 to 0.82 lambda, 35 psi boost on a Precision 6766, no knock. It was originally running 0.75 lambda but there was a slight ignition misfire which is why he leaned it out some. After leaning it out there was no misfire but it obliterated one of the spark plugs (with piston damage of course). From my understanding this type of sparkplug damage is normally always due to pre-ignition. Do you think the AFR was too lean for E98 or just too hot of a sparkplug (NGK #7).
Hi, the problem with obliterated one of the spark plugs is too hot of a sparkplug (NGK #7). Minimum for that setup NGK 9.
Agreed, you need to be running a colder plug than a 7. I normally run 8's for a street application and a 9 or 10 for a drag/race application.
The target lambda on E85/E98 is similar to what I aim for on pump fuel so you should have been in the ball park at 0.82. This is possibly marginally on the lean side and you might be better running around 0.80 - You won't lose any power but it will help reduce combustion temperature.
Thanks both of you for your responses. The head was pulled yesterday and the rod was bent at the small end and the wrist pin sheared on one side so this confirms to me that it was pre-ignition. The piston collapsed (loose in the bore), however there is no visible evidence of detonation or overheating on the top surface or ring land area. Is this typical in a pre-ignition scenario?
Normally pre ignition will show signs on the piston crown and in extreme cases pre ignition can very quickly blow a hole through the centre of the piston. It really depends on how long the engine was suffering pre ignition. Typically damage from pre ignition is worse and happens faster than detonation, however it is also dependent on specific power levels and how long the engine was subjected to pre ignition.
Ethanol as a fuel is much less likely to suffer from detonation but surprisingly is much more likely to suffer from pre ignition, particularly when run excessively rich. Of course a spark plug that's too hot also won't have helped your situation.
Andre thank you for clarification