Why Lean Engines DO NOT Run Hot video

[PT20J]

I ran across this video and I think he did a really good job of explaining the effect of different air/fuel ratios on engine operation.

 

[carusoam]

Excess air makes a great coolant... 

Just not as good as excess fuel...

Best regards,

-a-

[PT20J]

14 hours ago, carusoam said:

Excess air makes a great coolant... 

Just not as good as excess fuel...

Best regards,

-a-

I've been reading Prof. Heywood's textbook Internal Combustion Engine Fundamentals. Lot's of fascinating stuff in there. Combustion is a pretty complex phenomena. It's not so much that the air acts as a coolant for lean mixtures but the fact that there isn't enough fuel present to burn all the oxygen. This means that the engine is putting out less power than at richer mixtures and so there is less heat. The excess oxygen ensures that all the fuel is burned and this is what causes the increased fuel efficiency (in rich mixtures, the unburned fuel goes out the exhaust and is wasted). 

Skip

[N201MKTurbo]

Exactly right ^^^

Anything rich of peak is cooler than peak. Anything lean of peak is cooler than peak. The difference is, if you are rich of peak, you are putting $$$ out the exhaust port, if you are lean of peak you are putting air out the exhaust port.

I'm sure most people think lean is hotter because they always operate rich of peak, in which case leaner is hotter, until you go over the peak and then leaner is cooler.

[Warren]

I remember that book from my engineering classes.  Probably still have a copy somewhere.

The best efficiency is achieved slightly lean of peak and is represented on most graphs as bsfc (brake specific fuel consumption).  This means that lean of peak, the engine is at its most efficient operating point to convert fuel to power.  When we move from rich to lean operation, we do lose power as peak power assumes full combustion of the air introduced into the engine and requires some excess fuel to fully burn.  When we move to lean, we have excess air and thus make less power -- if we leave the air consumption fixed.

In a naturally aspirated engine, throttle can be increased to add additional air until max throttle is reached, and in a turbocharged engine, there is more air available to a higher altitude.  So, it is possible to add additional air and the corresponding fuel to increase or recover the power output while maintaining the higher fuel efficiency of lean operations.  Naturally aspirated engines quickly run out of air as they climb, thus limiting power quicker for lean operations than for rich.  Turbocharged engines have the ability to maintain power to higher altitudes but are usually limited by TIT.

[exM20K]

1 hour ago, PT20J said:

The excess oxygen ensures that all the fuel is burned and this is what causes the increased fuel efficiency (in rich mixtures, the unburned fuel goes out the exhaust and is wasted). 

.... and ensures the exhaust is free of CO

[jlunseth]

Well, almost. There is one significant difference between LOP and ROP operations, and that is that, in operation, the rich mixture can and needs to get much further away from peak than the lean mixture. The difference in our aircraft engines is very roughly 100 dF, in other words a mixture that is -25dF from peak is approximately equal in operational terms to one that is +125 dF. The CHTs resulting from that lean mixture will be cooler than those of the roughly equivalent rich mixture but the EGTs will be about 100 dF hotter, and for those of us who operate turbos the TIT will be hotter also. In my aircraft, this applies in practice up to a certain point in the air column where lack of cooling becomes an issue.When LOP started to become popular there was alot of talk about the effect on the valves of running constantly with higher EGTs, I have to say that in my own engine I have not noticed faster valve deterioration, in fact the valves all look cleaner and healthier after several hundred hours of mostly LOP cruise than  when I was running ROP. The TIT does become a limiting issue, I have noticed that at least in my engine it is difficult to keep the TIT under 1600 above some altitude, that altitude varying with time of year and OAT. My strong suspicion is that my baffling is leaking and preventing higher altitude LOP ops. However, if I keep TIT at or under 1600 that seems to have no deleterious effect on the turbocharger, mine is now about 1200 hours from the last time it was replaced, was borescoped at the last annual, and I was told it is in good shape and does not need servicing or replacement.

My point is that in theory engines run just as cool LOP as they do ROP, but in practice, some parts of the engine (i.e. the block and CHTs) are cooler and some hotter (the head, valves, exhaust, and if present, a turbo) when LOP and vice versa when ROP. It does appear that the components that run hotter LOP are well able to withstand the greater heat and may in fact benefit from it, or at least from the fact that they are not being subjected to unburned fuel.

[tmo]

@jlunseth - at what power setting is the 1600F TIT limiting for you?

I'm very curious, because my plane (stock M20K) seems to be happy to do 35/2500 at around 10.5 gph and below 1400 TIT and I don't know if I should just be happy, or worried that something isn't reading right. Granted this is fairly low (6500 ASL) relatively cold OAT (10C below ISA) and open cowl flaps (otherwise the CHT goes up enough to make me feel uneasy). I really need to get that EDM installed!

Your expert input is most appreciated!

[jlunseth]

My power setting for LOP cruise is 34"/2450 RPM/11.1 GPH, cowl flaps closed, which yields 72% HP. During the warm weather months and depending on OAT my TIT will start to creep over 1600 at somewhere around 12-16,000 ft. I just go ROP if I am going to cruise at a high altitude and cannot keep the TIT at about 1600 or under. Your setting is about 4 percentage points of HP lower than mine, about 68.5%. If the TIT starts to creep up over 1600 my first move is to reduce the fuel flow just a little but if I go too much below 11 GPH I start to lose too much speed for me, so I do something else (usually go ROP). This winter I am going to work on cooling. The baffles toward the rear of the engine do not smoothly engage the cowling and I think I have some leaks. My #2 cylinder (rearmost cylinder) tends to run hotter than the others by a fair amount and there appears to be a ripple in the baffle there. So that may be the reason.

On some hot summer days and at higher altitudes I may open the cowl flaps to the first notch, which helps with the cooling, but I don't like the speed loss. There is so much speed loss with cowl flaps full open I generally do not cruise that way. Bear in mind many of my trips in the upper midwest of the US are 600 nautical miles or more so additional speed is helpful.

If I want to use a lower power setting, say of around 120 kts. for an approach or hold speed, I make my lean setting at cruise and from there just pull the throttle back to about 24.5" MP without changing the fuel knob. The fuel system in the 231 automatically reduces the fuel flow to maintain roughly the same air/fuel ratio that was set at cruise. That yields a fuel flow of about 8.8 GPH at 24.5", which is about 57% HP. If I want to hit 120 exactly, the throttle setting will be somewhere around 24.5 but will vary some depending on OAT. Dropping the gear, putting in 10 degrees of flaps, and pulling the throttle back to about 19" (leaving the fuel knob untouched from cruise) will give me about 90 kts. down the glide slope of an instrument approach, although I find I have to drop the MP to about 14 or 15 for a little while to get the aircraft to slow down at the beginning of the slope, and then put in the 19-20" for the rest of the way down the slope.

Your power setting looks just fine to me, especially at low altitude and cool OATs. I have not found that there is much difference between 34 and 35" when LOP, which is why I generally use 34". Power is dictated by fuel flow when LOP, not by MP. You are a little leaner at 35" of course (more air in the air/fuel mix). I have a JPI 930 so I know exactly what is going on with each cylinder and with the overall engine and my aircraft had GAMI injectors installed before I bought it, so fuel flows to each of the cylinders are reasonably well balanced.

[PT20J]

Greek symbol phi is used by engineers to represent the fuel-air equivalence ratio. It is the fuel-air ratio divided by the stoichiometric fuel-air ratio. Thus, a stoichiometric mixture has a phi = 1.0. Phi > 1 is rich and phi < 1 is lean. The advantage of this is that different fuels having different stoichiometric ratios can easily be compared on a common scale.

For various reasons, an internal combustion engine never completely uses all the available fuel and oxygen at phi = 1, and that’s why best power occurs rich of peak at a point where all the fuel is burned. 

It helps me think about engine operation when I remember that the engine is primarily an air pump, but all the power comes from the fuel.

Automobiles often are set to run at phi = 1 because the catalytic converter is happiest there. For autos, emissions are more important than economy. A lot if economy is already built in since it takes very little power to maintain highway speeds. 

 

Skip

[larrynimmo]

So true skip

[jlunseth]

17 hours ago, PT20J said:

Greek symbol phi is used by engineers to represent the fuel-air equivalence ratio. It is the fuel-air ratio divided by the stoichiometric fuel-air ratio. Thus, a stoichiometric mixture has a phi = 1.0. Phi > 1 is rich and phi < 1 is lean. The advantage of this is that different fuels having different stoichiometric ratios can easily be compared on a common scale.

For various reasons, an internal combustion engine never completely uses all the available fuel and oxygen at phi = 1, and that’s why best power occurs rich of peak at a point where all the fuel is burned. 

It helps me think about engine operation when I remember that the engine is primarily an air pump, but all the power comes from the fuel.

Automobiles often are set to run at phi = 1 because the catalytic converter is happiest there. For autos, emissions are more important than economy. A lot if economy is already built in since it takes very little power to maintain highway speeds. 

 

Skip

Many pilots do not understand that this is what LOP or ROP mean, a fuel/air ratio that is leaner or richer than stoichiometric. They get tangled up with the way we measure those things in our aircraft, which is an indirect means; we are not measuring the fuel/air mixture by sampling and directly measuring it, we measure its effects - whether EGT on the cylinder we choose as the marker is down from peak by “x” degrees. It is entirely possible to cause that effect without changing the fuel/air ratio at all. One could, for example, find peak at a setting for, say, 75% HP, then bring the manifold pressure back, not the mixture. With many of our engines the control system will bring the mixture back automatically when the pilot reduces MP. The EGT will drop  and the pilot will tell himself that he is now “x” degrees LOP, that phi<1 as Skip puts it, when in fact the fuel/air ratio has not changed at all. Rather, the entire power setting has changed and the engine is now putting out less power, thus generating less heat. Phi still equals 1. Your engine monitor may be telling you that the engine is now “x” degrees lean of peak but that is not true, it is still at peak. Thankfully, in that instance you may have moved the power setting below 65% HP in which case it does not matter whether you are LOP, ROP, or at peak, but you are not lean of peak, you just think you are. If your power setting is more than 65% and you have done this, then you are operating the engine right where you do not want to.

The process is fairly simple in an NA engine. If you leave the throttle WOT and change the mixture the MP in most engines will stay at ambient (remain fixed) unless you change altitude. In the turbo it is less simple because, depending on wastegate controller type and power setting interconnections, if you lean the mixture out from the peak you found many things start to happen. Reduced power means less exhaust to the turbo, means the turbo spins slower and puts out less air pressure, which means that the wastegate controller needs to close the wastegate some in order to maintain a fixed MP which happens with varying degrees of accuracy depending on the controller. Engine control interconnects may kick in, changing everything.The engine control interconnects in most of our engines were designed in the “ROP only” era, so their operation assumes the engine is operating rich of peak, the interconnects may not move things as you would want them to move while the engine is LOP.

Our engine monitors assume an “all other things remain equal” status, in other words, that you change only mixture while RPM and MP remain exactly the same, but if that is not the case, then what the monitor reports for degrees LOP or ROP is not accurate.

My point is simple, unless you start and end the process at the same MP and RPM, pulling the mixture back is not changing the fuel/air ratio your engine monitor is reporting, it is just changing the power setting regardless of what your monitor says.

[ArtVandelay]

I thought that running lean slows the combustion process, so requires advanced timing. Basically it takes longer for the fuel molecules to pair up with the oxygen molecules. And if combustion process slows and results in burning more fuel after TDC this is what generates more heat (not the phase change of fuel or excess air) as explained in the video).

[Bob - S50]

32 minutes ago, ArtVandelay said:

I thought that running lean slows the combustion process, so requires advanced timing. Basically it takes longer for the fuel molecules to pair up with the oxygen molecules. And if combustion process slows and results in burning more fuel after TDC this is what generates more heat (not the phase change of fuel or excess air) as explained in the video).

The way I look at it.

Running either ROP or LOP will slow the combustion process, which moves peak pressure further past TDC which reduces CHT.

At peak EGT, because the fuel air mixture is not perfectly distributed, there is both unused oxygen and partially burned fuel going out the exhaust.

By about 75F ROP, I've added enough extra fuel to effectively use up all the oxygen without significantly slowing the combustion process so I get maximum power.  I've now got even more partially burned fuel going out the exhaust but essentially zero excess oxygen.  Any rich than that and I'm just wasting fuel unless I'm doing it on purpose to avoid detonation (full power takeoffs/climbs for example).

By about 30 - 50F LOP, I've done just the opposite.  I've now essentially used up all the fuel with more unused oxygen without significantly slowing the combustion process.  Any leaner than that and I'm losing power for two reasons: less fuel to burn and slower burn rate, both of which reduce CHT.

[ArtVandelay]

The way I look at it.
Running either ROP or LOP will slow the combustion process, which moves peak pressure further past TDC which reduces CHT.

Which is bad because we’ll lose power if peak pressure is past TDC, we want peak pressure just before TDC. I wonder what the power curve would look with variable timing.

[Bob - S50]

2 hours ago, ArtVandelay said:

Which is bad because we’ll lose power if peak pressure is past TDC, we want peak pressure just before TDC. I wonder what the power curve would look with variable timing.

Uhhh, no.  If peak pressure is before TDC the engine will try to run backwards.  Ideally peak pressure will happen about 15 or 16 degrees AFTER TDC.

[ArtVandelay]

Uhhh, no.  If peak pressure is before TDC the engine will try to run backwards.  Ideally peak pressure will happen about 15 or 16 degrees AFTER TDC.

Got it.

[PT20J]

It takes a while for the pressure to build to a peak after the spark ignites it, and then pressure decreases until the exhaust valve opens. All the pressure increase during compression (that is before TDC) represents negative work -  not useful work. So, you want to minimize that by having the peak pressure occur ATDC. But if peak pressure occurs too late, the piston is already accelerating down the bore and the pressure produces less useful work. Obviously, there must be an optimum somewhere in between too early and too late. When the spark advance is optimized to produce maximum brake torque (MBT) the peak pressure is usually about 16 deg. ATDC.

Skip

[Raymond J1]

The unhealthy side of the LOP exists, it is a lack of temperatures to "finish" the reduction of dioxides. The engine is only a converter, and its problem is thermal instability. Beyond a threshold, we find it difficult to run LOP because our air-cooled engines do not have the right metallurgy. This is not a problem for liquid-cooled engines, the "Lancasters" used LOP as early as 1942, by the way, this was the only way to go so far into hostile territory. This question seems insoluble for Lycoming and Continental engines while it is a non-event for a manufacturer like Rotax whose products nevertheless develop specific powers higher than that of a 550 cu.inch. The work of the gami team is sensational, but I have to say that in Europe the discourse seems old, it feels like they are rescuing an old and lost driving. As you know, the fuel E 100 (100% ethanol) at an octane number above 100 LL Avgas, our question is rather : when is E100 allowed on IO 360 or IO 550 of M20 by Lycoming and Continental ?

[carusoam]

Ethanol challenges are solubility with rubber/polymer parts...

And has a tendency to hold more moisture in solution... which can be challenging for metals that want to rust... (I think we might have eliminated most of those things, but a review will be needed.

Be sure to check how well your seals, sealant, pumps, diaphragms, and fuels are at resisting being destroyed by the different chemistries...

Kind of opens an old can of worms... 

+1 for 100 octane alternatives...

+2 for lead free...

PP thoughts only, not a chemist...

Best regards,

-a-