The understanding of EGT in diesel applications is lower and petrol applications is higher causes a degree of confusion. In an attempt to clarify for the purpose of education.
I hope you will enjoy it as I did.
Cummins Express
11-16-2005, 12:33 AM
Okay, so I will try to make this as short and to the point as possible (for me anyways) without sacrificing key points or in the opposite, open doors for controversy. So why do EGT’s behave differently between gasoline and diesel engines? First let’s look at both engines as air pumps, as they are, fundamentally. From here, let’s look at their means of power control.
Gas engines rely on a throttle air valve to control the power delivery. Whether it be an MPI, throttle body, TPI, or good old fashioned carburetor, throttle plate angle dictates fuel delivery whether a throttle position sensor attached to a butterfly, or a booster venturi responding to air flow through a carburetor venturi. So….fuel is metered to correspond to the engine’s air ingestion. Nothing new right? Okay, diesel engines have no throttle valve, or any means to control air ingestion. Their intake manifold is more or less “wide open”. You’ve heard diesel engines don’t build vacuum right? Well they still draw in air the same way a gas engine does. However, it’s the closed, or partially closed, throttle plate restricting a gas engine’s air ingestion that creates the vacuum. Therefore, without a throttle air valve, no vacuum can be built. Back on track….so instead of metering air, a diesel meters it’s fuel through a high pressure fuel injection pump, which distributes fuel to each cylinder as a direct injection fuel system controlled by a throttle cable or TPS depending on whether it be electronically injected or mechanically. The new breed of diesel injection incorporates a high pressure (35,000 psi) common rail system, which I won‘t go into because it has no bearing on this post. So already, the differences are mounting between the two engines. Next compression ratios run in the 17:1 range in diesels and conversely 8-11:1 in street gas engines. Why the high CR? Air temperature. Diesels ignite via the air charge being heated by compression to a point where a finely atomized stream of fuel will self ignite. And we all know how gas engine ignite right…..
So why does all of this matter you ask? Because it is important to realize the fundamental differences between the two engines, even though generally, they both start life as a 4 stroke, overhead valve reciprocating group of cylinders with pistons in them. We all know what stoich stands for right? Stoichiometric, or a 14.7:1 air/fuel ratio. You will find gas engines to be at peak EGT’s near a perfect stioch burn. While this is not the perfect ratio in MANY instances for power, it will be where peak EGT’s occur, and where peak engine efficiency lives. That is because the combustion process is in a perfect equilibrium and all air and fuel is consumed. Peak EGT will always occur at stoich. They will not climb higher with more fuel, and they will not climb higher in a lean condition either. Gas engine WILL reach a peak and drop off to one side or the other depending on tune. This is where gasoline differs from diesel. Diesel EGTs will continue to climb as a rich condition becomes richer with added fuel and no added air mass. So when tuning a gas engine, we look at a plot from our loggers, we see EGT’s high at one rpm but low at another. This is the engine’s combustion requirements and characteristics changing and shows a need to program the injection system and ignition to compensate. Generally speaking, your engine will be at it’s best power production on the cooler, rich side of peak EGT, and consequently the stoich A/F. An engine will be at peak economy at peak EGT. Some of you guys are screaming timing, knock, and boost rate and so forth I know, but let’s keep it simple. You still need to find a fuel baseline, and a wide band O2 in conjunction with a pyro will do this. Then play with timing and fine tune the fuel and boost to stay out of knock. Boost adds many more variables that I don’t want to go into. We’ll just assume a safe, average boost level. And we won’t talk alcohol either, other than it will raise the effective octane rating of gasoline, hence making it more resistive to detonation, which is a powerful aid in tuning for maximum boost and consequently performance. Even an N/A engine will enjoy the same benefits with alky by being able to run a deeper spark advance to try and get the engine's spark to the cylinder at that perfect 17* ATDC, which is the most effective leverage point the piston and rod have on a crankshft journal. Okay, so if we see lower than our now known peak EGT’s does this automatically mean we are on the rich side of stoich? NO!! Additionally, many people think that the leaner you go, the higher the EGT gets. This is also incorrect. Peak EGT occurs at stoichiometry. If you go richer than stoich, EGT will drop and by the same token if you go leaner than stoich, EGT will ALSO drop due to the hindrance of the combustion process from lack of fuel in relation to air mass. It is VERY important to know which side of peak EGT you are on before making adjustments. It is safe to say that peak power will, however, occur at an EGT somewhat colder than peak EGT. So adding fuel will show a temp decline on the pyro. Why? Quite simply, the extra fuel acts to cool the cylinder, as well as cause an imbalance from the perfect A/F ratio, thus ending in a less than peak combustion event….hence a lower EGT. By adding fuel, however, one must pay attention to knock, and adjust fuel and timing curves (or alcohol rate) as well as boost #s to maintain a detonation free engine.
Now Diesels. There is no theoretic stoich condition for diesel combustion. They may run as lean as 100:1 at idle or as rich as 3:1 at full power. Unlike a gasoline motor, a diesel motor will continue to make power as more fuel is added. As more fuel is added, heat will be generated until the motor just gets too hot and things start to melt. We don't run into knock syndrome either with overfueling, as diesel fuel is ignited by temperature ignition anyway. Remember too, that diesels regulate power and rpm by changing the amount of fuel injected, not by throttle angle with a butterfly but by changing a relative "rack" position via a direct coupled throttle cable. Diesel EGT readings can be as low as 200* at idle to a maximum peak of 1300* at full load and power. Generally, 1250* is considered the most efficient sustained EGT, as well as close to redline temperature. Higher temps CAN be sustained for short intervals, but at a danger to pistons and turbine wheels. And the damage is cumulative, the more you do it, the worse the damage becomes, until the pistons melt down that is, or the turbine wheel deforms to the point of explosion or inoperation. Another interesting note is that diesel fuel per pound releases around 145K BTUs, whereas Gasoline releases around 120K BTUs per pound of fuel. While gasoline is more volatile, diesel contains more heat energy, and will sustain that heat for a longer interval. The injection event continues far past TDC, and gas expansion and thermal development remains right down to BDC. As more of this heat energy is released through heavier fueling, high CR’s, and extreme cylinder pressures (as high as 190,000 psi), EGTs can soar quickly at a given, constant, laminar charge air supply. If we were to pull fuel in relation to that air volume, heat energy would be reduced and consequently EGT’s. But nobody, including me, wants to pull that fuel away from a powerful engine just becasue it gets hot at high loads in high gears. That fuel level provides high power at lower engine loads ie lower gears. So we look to the next best thing…add more air!! Not necessarily more boost, but more mass, aka lbs of air. Cummins diesels can gobble up as much as 2000 cfm, or more than 150 pounds of air per minute at high pressure ratio levels, if we could get the charge air temps within tolerable specs that is, which is where huge after coolers and king kong sized compressors are key to moving huge volumes of air without overheating it through the compression process. So we share the skills of tuning a diesel via EGTs just like the gas boys do. We merely add air volume, or reduce fuel rate to lower peak EGTs instead of adding fuel and decreasing boost like gas tuning involves. There are a few other tuning areas where we share the same ideology. Timing can have a large influence on EGTs, though much less forgiving and way more precise. We are timing the fuel injection event, rather than the spark event. A 1 degree timing change is a large adjustment, and can mean as little as 2/10 of a millimeter in relation to injection pump plunger lift, which is what we’re actually changing when we time diesels. But it can also drop a few hundred degrees off of peak EGT, and mean as much as 3 mpg fuel efficiency gain. Obviously, specialized dial indicators are required here!! A timing increase will generally lower EGTs, but at the sacrifice of low end torque. This loss will be made up for however in upper rpm performance. A 50/50 solution of methanol and water will also drop a few hundred degrees off, however care must be taken to not wash the cylinders, or inject it at low boost, or to let it dribble from a damaged jet, or a low supply pump pressure problem. Non-atomized fuids will absolutely cause havoc in a diesel. In addition to cooling the cylinders, water/meth will generally act as an alternative fuel by adding as much as 80 hp at full pressure, as well as keep carbon deposits cleaned out of the cylinders and exhaust ports.
That’s about all I’m good for tonight. There are a lot of holes yet to fill, and chemical equations and scientific explanations for the differences between the two fuels and how they behave in our engines. But you’ll have to draft the help of someone smarter, and with more time than me to answer those phenomenon. After all, I’m just a good ol’ #2 fuel oil sniffin’ grease monkey whos knuckles bleed as well as the next guy‘s!!
The diesel EGT of 1250F is preturbo.
A good read on EGT and placement of the sensor is here.
Banks Power | Why EGT is Important
Jim
I hope you will enjoy it as I did.
Cummins Express
11-16-2005, 12:33 AM
Okay, so I will try to make this as short and to the point as possible (for me anyways) without sacrificing key points or in the opposite, open doors for controversy. So why do EGT’s behave differently between gasoline and diesel engines? First let’s look at both engines as air pumps, as they are, fundamentally. From here, let’s look at their means of power control.
Gas engines rely on a throttle air valve to control the power delivery. Whether it be an MPI, throttle body, TPI, or good old fashioned carburetor, throttle plate angle dictates fuel delivery whether a throttle position sensor attached to a butterfly, or a booster venturi responding to air flow through a carburetor venturi. So….fuel is metered to correspond to the engine’s air ingestion. Nothing new right? Okay, diesel engines have no throttle valve, or any means to control air ingestion. Their intake manifold is more or less “wide open”. You’ve heard diesel engines don’t build vacuum right? Well they still draw in air the same way a gas engine does. However, it’s the closed, or partially closed, throttle plate restricting a gas engine’s air ingestion that creates the vacuum. Therefore, without a throttle air valve, no vacuum can be built. Back on track….so instead of metering air, a diesel meters it’s fuel through a high pressure fuel injection pump, which distributes fuel to each cylinder as a direct injection fuel system controlled by a throttle cable or TPS depending on whether it be electronically injected or mechanically. The new breed of diesel injection incorporates a high pressure (35,000 psi) common rail system, which I won‘t go into because it has no bearing on this post. So already, the differences are mounting between the two engines. Next compression ratios run in the 17:1 range in diesels and conversely 8-11:1 in street gas engines. Why the high CR? Air temperature. Diesels ignite via the air charge being heated by compression to a point where a finely atomized stream of fuel will self ignite. And we all know how gas engine ignite right…..
So why does all of this matter you ask? Because it is important to realize the fundamental differences between the two engines, even though generally, they both start life as a 4 stroke, overhead valve reciprocating group of cylinders with pistons in them. We all know what stoich stands for right? Stoichiometric, or a 14.7:1 air/fuel ratio. You will find gas engines to be at peak EGT’s near a perfect stioch burn. While this is not the perfect ratio in MANY instances for power, it will be where peak EGT’s occur, and where peak engine efficiency lives. That is because the combustion process is in a perfect equilibrium and all air and fuel is consumed. Peak EGT will always occur at stoich. They will not climb higher with more fuel, and they will not climb higher in a lean condition either. Gas engine WILL reach a peak and drop off to one side or the other depending on tune. This is where gasoline differs from diesel. Diesel EGTs will continue to climb as a rich condition becomes richer with added fuel and no added air mass. So when tuning a gas engine, we look at a plot from our loggers, we see EGT’s high at one rpm but low at another. This is the engine’s combustion requirements and characteristics changing and shows a need to program the injection system and ignition to compensate. Generally speaking, your engine will be at it’s best power production on the cooler, rich side of peak EGT, and consequently the stoich A/F. An engine will be at peak economy at peak EGT. Some of you guys are screaming timing, knock, and boost rate and so forth I know, but let’s keep it simple. You still need to find a fuel baseline, and a wide band O2 in conjunction with a pyro will do this. Then play with timing and fine tune the fuel and boost to stay out of knock. Boost adds many more variables that I don’t want to go into. We’ll just assume a safe, average boost level. And we won’t talk alcohol either, other than it will raise the effective octane rating of gasoline, hence making it more resistive to detonation, which is a powerful aid in tuning for maximum boost and consequently performance. Even an N/A engine will enjoy the same benefits with alky by being able to run a deeper spark advance to try and get the engine's spark to the cylinder at that perfect 17* ATDC, which is the most effective leverage point the piston and rod have on a crankshft journal. Okay, so if we see lower than our now known peak EGT’s does this automatically mean we are on the rich side of stoich? NO!! Additionally, many people think that the leaner you go, the higher the EGT gets. This is also incorrect. Peak EGT occurs at stoichiometry. If you go richer than stoich, EGT will drop and by the same token if you go leaner than stoich, EGT will ALSO drop due to the hindrance of the combustion process from lack of fuel in relation to air mass. It is VERY important to know which side of peak EGT you are on before making adjustments. It is safe to say that peak power will, however, occur at an EGT somewhat colder than peak EGT. So adding fuel will show a temp decline on the pyro. Why? Quite simply, the extra fuel acts to cool the cylinder, as well as cause an imbalance from the perfect A/F ratio, thus ending in a less than peak combustion event….hence a lower EGT. By adding fuel, however, one must pay attention to knock, and adjust fuel and timing curves (or alcohol rate) as well as boost #s to maintain a detonation free engine.
Now Diesels. There is no theoretic stoich condition for diesel combustion. They may run as lean as 100:1 at idle or as rich as 3:1 at full power. Unlike a gasoline motor, a diesel motor will continue to make power as more fuel is added. As more fuel is added, heat will be generated until the motor just gets too hot and things start to melt. We don't run into knock syndrome either with overfueling, as diesel fuel is ignited by temperature ignition anyway. Remember too, that diesels regulate power and rpm by changing the amount of fuel injected, not by throttle angle with a butterfly but by changing a relative "rack" position via a direct coupled throttle cable. Diesel EGT readings can be as low as 200* at idle to a maximum peak of 1300* at full load and power. Generally, 1250* is considered the most efficient sustained EGT, as well as close to redline temperature. Higher temps CAN be sustained for short intervals, but at a danger to pistons and turbine wheels. And the damage is cumulative, the more you do it, the worse the damage becomes, until the pistons melt down that is, or the turbine wheel deforms to the point of explosion or inoperation. Another interesting note is that diesel fuel per pound releases around 145K BTUs, whereas Gasoline releases around 120K BTUs per pound of fuel. While gasoline is more volatile, diesel contains more heat energy, and will sustain that heat for a longer interval. The injection event continues far past TDC, and gas expansion and thermal development remains right down to BDC. As more of this heat energy is released through heavier fueling, high CR’s, and extreme cylinder pressures (as high as 190,000 psi), EGTs can soar quickly at a given, constant, laminar charge air supply. If we were to pull fuel in relation to that air volume, heat energy would be reduced and consequently EGT’s. But nobody, including me, wants to pull that fuel away from a powerful engine just becasue it gets hot at high loads in high gears. That fuel level provides high power at lower engine loads ie lower gears. So we look to the next best thing…add more air!! Not necessarily more boost, but more mass, aka lbs of air. Cummins diesels can gobble up as much as 2000 cfm, or more than 150 pounds of air per minute at high pressure ratio levels, if we could get the charge air temps within tolerable specs that is, which is where huge after coolers and king kong sized compressors are key to moving huge volumes of air without overheating it through the compression process. So we share the skills of tuning a diesel via EGTs just like the gas boys do. We merely add air volume, or reduce fuel rate to lower peak EGTs instead of adding fuel and decreasing boost like gas tuning involves. There are a few other tuning areas where we share the same ideology. Timing can have a large influence on EGTs, though much less forgiving and way more precise. We are timing the fuel injection event, rather than the spark event. A 1 degree timing change is a large adjustment, and can mean as little as 2/10 of a millimeter in relation to injection pump plunger lift, which is what we’re actually changing when we time diesels. But it can also drop a few hundred degrees off of peak EGT, and mean as much as 3 mpg fuel efficiency gain. Obviously, specialized dial indicators are required here!! A timing increase will generally lower EGTs, but at the sacrifice of low end torque. This loss will be made up for however in upper rpm performance. A 50/50 solution of methanol and water will also drop a few hundred degrees off, however care must be taken to not wash the cylinders, or inject it at low boost, or to let it dribble from a damaged jet, or a low supply pump pressure problem. Non-atomized fuids will absolutely cause havoc in a diesel. In addition to cooling the cylinders, water/meth will generally act as an alternative fuel by adding as much as 80 hp at full pressure, as well as keep carbon deposits cleaned out of the cylinders and exhaust ports.
That’s about all I’m good for tonight. There are a lot of holes yet to fill, and chemical equations and scientific explanations for the differences between the two fuels and how they behave in our engines. But you’ll have to draft the help of someone smarter, and with more time than me to answer those phenomenon. After all, I’m just a good ol’ #2 fuel oil sniffin’ grease monkey whos knuckles bleed as well as the next guy‘s!!
The diesel EGT of 1250F is preturbo.
A good read on EGT and placement of the sensor is here.
Banks Power | Why EGT is Important
Jim
