Stall speed of torque converter in Allison bus transmission?
Posted: Fri May 29, 2009 12:40 pm
I'm going to present just a very simplified technical discussion FIRST, and then ask my quesiton so readers can understand WHY I am asking the question.
Most (not all!) "automatic" transmissions use a torque converter along with the transmission itself. In our Eagle buses, those of us who have automatic transmissions all have one model or another of an Allison transmission, and all have a torwue converter. This serves at least 2 important needs:
1. The bus can be at a stop with the engine running and in gear, without stalling the engine
2. The torque converter is designed to "slip" at low engine rpm, allowing the engine to "wind up" a bit to a higher rpm despite the slow, or even "zero mph" current vehicle speed.
This second function is important because it enables two really good things:
1. The engine is allowed to spin faster than it otherwise would for the current vehicle speed. This increases the power available, since most diesel engines have either flat or INCREASING torque curves at low rpm, and power produced = Torque x RPM / 5250. So, the greater the rpm, the greater the available power
2. When the torque converter "slips", i.e. when its rpm does not match the engine's rpm despite their 1 to 1 nominal coupling, it multiples the engine's crankshaft torque by a lot - up to about 3 times - depending upon variables of torque converter design. This enables a bus to apply more torque, and thus power, to get moving on uphills, brake out of mud, etc. The only negative associated with this is that slipping produces a lot of HEAT, so if the torque converter is allowed to slip too long, the entire torque converter and transmission assembly overheats and then self-destructs. So, moderation is key.
Above a certain engine rpm, which again depends upon the design of the torque converter and the engine torque, there is no more "slip". The torque converter rpm = engine rpm for that engine rpm and all higher engine rpms. That specific rpm is called the "stall speed" of the torque converter. This is admittedly simplified a bit, as the more technical readers will know.
Now to get to the reason for my question:
I have a really good, proven performance computer model that I have used for many years when modeling vehicle drivetrain performance for cars, trucks, and motorcycles. It generally gives me pretty accurate results, and is very helpful when trying to optimize things like rear axle gearing, and engine dyno curves for given circumstances. However, all the accumulated historical data in it is for cars, trucks, and motorcycles, not buses.
I do have torque converter stall speeds for many different vehicles I have modeled over the past couple of decades in it, but the stall speeds for cars and trucks, especially for the PERFORMANCE cars and trucks I have modeled and optimized, are radically different than the ones for buses! To give you just one example, the stall speed for a factory stock Chevrolet SSR retro pickup was about 1800 rpm, which is pathetically low for a performance vehicle, especially a vehicle that is HEAVY (4800 lb or so in the case of the SSR) with only a 5.3 liter gasoline engine. The souped up SSR ended up with a stall speed that was much higher - too high in fact (because the engine's final power output exceeded early expectations by a lot): about 3700 rpm. Its pretty typical for performance car (gasoline) engines, whose useful power ranges often run from about 2000 rpm to about 6000 rpm, to have converter stall speeds in the 2200 to 3000 rpm range.
However, a bus spends most of its cruising time at 1500 to 1900 rpm (with the lower end of the range being many of the 4-stroke engines, and the higher end being the 2-stroke Detroit Diesel engines like in early Eagles). PEAK engine power is often restricted via the governor versus any obvious airflow or mechanical limitations) - the power is restricted simply by the governor mechanism limiting the engine rpm - and is often in the 1800 to 2100 rpm range. This is a MUCH lower rpm than for a typical gasoline engine car or truck, and so obviously the torque converter stall speed needs to be different as well.
Not knowing the stall speed is a problem for the modeling software. Without knowing it, it cannot accurately model the performance at low rpm.
Does anyone out there know what stall speeds are egnerally used on bus transmissions? Specifically, at least at first, what stall speeed would probably apply to the troque converter in my 1979 Eagle with 8V71N engine, which originally came from the factory with a manual transmission, but was retrofitted later with an Allison HT740? I say"probably" because I suspect that not all HT740 transmissions came with the exact same stall speed, but I am hoping that if someone put the HT740 in there, he or she had the good sense to select a BUS model HT740, versus one used in say a bulldozer. If he or she did not, obviously my little modeling experiment will have limited accuracy at lower rpm.
ANYONE out there have access to this kind of information?
If I can get the information, I can treat you all to a "virtual" (on the computer) 1/4 mile accceleration run by a 1979 Eagle conversion bus! THAT should be fun . . .
Most (not all!) "automatic" transmissions use a torque converter along with the transmission itself. In our Eagle buses, those of us who have automatic transmissions all have one model or another of an Allison transmission, and all have a torwue converter. This serves at least 2 important needs:
1. The bus can be at a stop with the engine running and in gear, without stalling the engine
2. The torque converter is designed to "slip" at low engine rpm, allowing the engine to "wind up" a bit to a higher rpm despite the slow, or even "zero mph" current vehicle speed.
This second function is important because it enables two really good things:
1. The engine is allowed to spin faster than it otherwise would for the current vehicle speed. This increases the power available, since most diesel engines have either flat or INCREASING torque curves at low rpm, and power produced = Torque x RPM / 5250. So, the greater the rpm, the greater the available power
2. When the torque converter "slips", i.e. when its rpm does not match the engine's rpm despite their 1 to 1 nominal coupling, it multiples the engine's crankshaft torque by a lot - up to about 3 times - depending upon variables of torque converter design. This enables a bus to apply more torque, and thus power, to get moving on uphills, brake out of mud, etc. The only negative associated with this is that slipping produces a lot of HEAT, so if the torque converter is allowed to slip too long, the entire torque converter and transmission assembly overheats and then self-destructs. So, moderation is key.
Above a certain engine rpm, which again depends upon the design of the torque converter and the engine torque, there is no more "slip". The torque converter rpm = engine rpm for that engine rpm and all higher engine rpms. That specific rpm is called the "stall speed" of the torque converter. This is admittedly simplified a bit, as the more technical readers will know.
Now to get to the reason for my question:
I have a really good, proven performance computer model that I have used for many years when modeling vehicle drivetrain performance for cars, trucks, and motorcycles. It generally gives me pretty accurate results, and is very helpful when trying to optimize things like rear axle gearing, and engine dyno curves for given circumstances. However, all the accumulated historical data in it is for cars, trucks, and motorcycles, not buses.
I do have torque converter stall speeds for many different vehicles I have modeled over the past couple of decades in it, but the stall speeds for cars and trucks, especially for the PERFORMANCE cars and trucks I have modeled and optimized, are radically different than the ones for buses! To give you just one example, the stall speed for a factory stock Chevrolet SSR retro pickup was about 1800 rpm, which is pathetically low for a performance vehicle, especially a vehicle that is HEAVY (4800 lb or so in the case of the SSR) with only a 5.3 liter gasoline engine. The souped up SSR ended up with a stall speed that was much higher - too high in fact (because the engine's final power output exceeded early expectations by a lot): about 3700 rpm. Its pretty typical for performance car (gasoline) engines, whose useful power ranges often run from about 2000 rpm to about 6000 rpm, to have converter stall speeds in the 2200 to 3000 rpm range.
However, a bus spends most of its cruising time at 1500 to 1900 rpm (with the lower end of the range being many of the 4-stroke engines, and the higher end being the 2-stroke Detroit Diesel engines like in early Eagles). PEAK engine power is often restricted via the governor versus any obvious airflow or mechanical limitations) - the power is restricted simply by the governor mechanism limiting the engine rpm - and is often in the 1800 to 2100 rpm range. This is a MUCH lower rpm than for a typical gasoline engine car or truck, and so obviously the torque converter stall speed needs to be different as well.
Not knowing the stall speed is a problem for the modeling software. Without knowing it, it cannot accurately model the performance at low rpm.
Does anyone out there know what stall speeds are egnerally used on bus transmissions? Specifically, at least at first, what stall speeed would probably apply to the troque converter in my 1979 Eagle with 8V71N engine, which originally came from the factory with a manual transmission, but was retrofitted later with an Allison HT740? I say"probably" because I suspect that not all HT740 transmissions came with the exact same stall speed, but I am hoping that if someone put the HT740 in there, he or she had the good sense to select a BUS model HT740, versus one used in say a bulldozer. If he or she did not, obviously my little modeling experiment will have limited accuracy at lower rpm.
ANYONE out there have access to this kind of information?
If I can get the information, I can treat you all to a "virtual" (on the computer) 1/4 mile accceleration run by a 1979 Eagle conversion bus! THAT should be fun . . .