Thermal Efficiency

[Shel_B]

Over the past year or more I have been hearing about Toyota's 41% thermal efficiency on some of its engines. I am unfamiliar with thermal efficiency, and it might be nice to have some discussion about it.

Just what is thermal efficiency and why is it important, if indeed it is? Toyota claims 41% TE, what happens to the remaining 51%? Is it very difficult to attain greater TE? What are some ways that might be done? Is there a downside to a greater percentage?

 

[Rand]

A quick google produced this: which is more elegant than how I was going to write it out.

Thermal efficiency is the ratio of the useful work output to the heat input of an engine. It measures how well an engine converts fuel energy into mechanical work. The higher the thermal efficiency, the less fuel consumption and emissions per unit of work.

 

[tnt_motorsports]

Over the past year or more I have been hearing about Toyota's 41% thermal efficiency on some of its engines. I am unfamiliar with thermal efficiency, and it might be nice to have some discussion about it.

Just what is thermal efficiency and why is it important, if indeed it is? Toyota claims 41% TE, what happens to the remaining 51%? Is it very difficult to attain greater TE? What are some ways that might be done? Is there a downside to a greater percentage?

The other 59% is lost to heat.

Basically TE is the percentage of the fuel turned into power. By nature combustion engines are not very efficient.

Just my $0.02

 

[supton]

Man, I miss college. In college I took some thermo classes, which was not related to my course of study, but I found it fascinating nevertheless. Now it's all over my head--wish I could just start over again in college, would be so much fun...

Anyhow. IIRC there is an upper limit to Carnot efficiency. Again, past me, but I don't think we can get much more from reciprocating engines? I'll leave it to people far wiser than me to explain it, using terms that a 5 year old can understand.

On the flip side, hitting 41% means that we're not far from an upper limit to mpg. Nothing will hit 100% so this kinda/sorta says we can never do more than about double the mpg that we are getting today. That's for an engine under continuous usage. A hybrid starts and runs the engine under high load, so when the engine is on, it's at this 41%. It's not 41% at idle, and it's probably not 41% at 100% WOT / max horsepower. By running in a pulse and glide sort of manner, hybrids eek out what there is to be had. It's going to be hard to get more I think. I could be wrong, but it takes a certain amount to overcome rolling losses, drivetrain losses, and air drag.

*

I believe in order to hit 41% efficiency they are running Atkinson cycle. Doing this means the engine gets less volumetric efficiency, which means less horsepower per liter. I suppose that is the downside. After that we can wring our hands about DI and 0W16 and electric water pumps and electrically controlled thermostats and electronically controlled oil pumps--all of which are being used to eek out all there is.

 

[Jetronic]

Man, I miss college. In college I took some thermo classes, which was not related to my course of study, but I found it fascinating nevertheless. Now it's all over my head--wish I could just start over again in college, would be so much fun...

Anyhow. IIRC there is an upper limit to Carnot efficiency. Again, past me, but I don't think we can get much more from reciprocating engines? I'll leave it to people far wiser than me to explain it, using terms that a 5 year old can understand.

On the flip side, hitting 41% means that we're not far from an upper limit to mpg. Nothing will hit 100% so this kinda/sorta says we can never do more than about double the mpg that we are getting today. That's for an engine under continuous usage. A hybrid starts and runs the engine under high load, so when the engine is on, it's at this 41%. It's not 41% at idle, and it's probably not 41% at 100% WOT / max horsepower. By running in a pulse and glide sort of manner, hybrids eek out what there is to be had. It's going to be hard to get more I think. I could be wrong, but it takes a certain amount to overcome rolling losses, drivetrain losses, and air drag.

*

I believe in order to hit 41% efficiency they are running Atkinson cycle. Doing this means the engine gets less volumetric efficiency, which means less horsepower per liter. I suppose that is the downside. After that we can wring our hands about DI and 0W16 and electric water pumps and electrically controlled thermostats and electronically controlled oil pumps--all of which are being used to eek out all there is.

Actually, it's more likely to be 41% at wide open throttle around the max torque RPM. Throttling causes a vacuum to be applied on a piston trying to move down, robbing power and efficiency.

 

[supton]

Actually, it's more likely to be 41% at wide open throttle around the max torque RPM. Throttling causes a vacuum to be applied on a piston trying to move down, robbing power and efficiency.

True, your are correct. But at high rpm, and thus high hp, I think the efficiency falls down a bit. But you are right, WOT, less pumping losses. I just wanted to make it known that it was not at like 5,000 rpm.

I've long wondered, on the older non-VVT engines, if max efficiency was at max torque. That's maximum applied force coming out. That is not maximum rate of work being done though. I don't know how that changes with VVT, other than, maybe rpm on those engines matters a bit less, any rpm in that wider range is ok.

 

[Jetronic]

True, your are correct. But at high rpm, and thus high hp, I think the efficiency falls down a bit. But you are right, WOT, less pumping losses. I just wanted to make it known that it was not at like 5,000 rpm.

I've long wondered, on the older non-VVT engines, if max efficiency was at max torque. That's maximum applied force coming out. That is not maximum rate of work being done though. I don't know how that changes with VVT, other than, maybe rpm on those engines matters a bit less, any rpm in that wider range is ok.

On VVT engines the efficiency doesn't change: they're still most efficient around max torque but the drop off in torque and efficiency isn't as bad when you deviate from max torque rpm. So quite important for car engines, not important for anything that runs a steady rpm.

BTW I had a car which reached max torque at 5000 rpm (but made a lot before that aswell, vvt on the intake 2 valves per cylinder - 1980s design) and max power at 6500 rpm. It was a blast to drive, and could be driven in top gear from idle (19 mph) up to redline. I did work on it a bit and made it produce power up to 6900 rpm: the car did 150 mph after, 130 mph when new. Some honda engines produce max torque at even higher rpm I believe but the ones I've driven are not pleasant to drive at low rpm.

 

[Shel_B]

Thanks to everyone who jumped in

 

[MSCH]

they used to say about 1/3 of the fuel consumed used to go to turning the wheels, 1/3 went out the tailpipe and the last third went into the cooling system.. not much has changed in all these years.

 

[John105]

We often don't think about efficiency.

Food for thought--why don't we simply run more cars on propane, it's clean?

I didn't know myself that propane has more energy than natural gas.

Ever had a gas furnace not work when it's single digits outside? I learned basically nothing can heat up the house. I even tried the gas oven and leaving it on (not smart but I tried to see). Tried to use portable electric ones, they are like a drop in the bucket and consuming much electricity.

Actually, the furnace in itself is a lesson. Our old one was 78%, and our new one 94%. Uses less natural gas to heat the home, sealed combustion. They went up to 96% at the time but we went with 94%....

 

[MSCH]

We often don't think about efficiency.

Food for thought--why don't we simply run more cars on propane, it's clean?

I didn't know myself that propane has more energy than natural gas.

Ever had a gas furnace not work when it's single digits outside? I learned basically nothing can heat up the house. I even tried the gas oven and leaving it on (not smart but I tried to see). Tried to use portable electric ones, they are like a drop in the bucket and consuming much electricity.

Actually, the furnace in itself is a lesson. Our old one was 78%, and our new one 94%. Uses less natural gas to heat the home, sealed combustion. They went up to 96% at the time but we went with 94%....

well For example, conventional gasoline has an energy content of 116,090 Btus per gallon, while propane has an energy content of 84,250 Btus per gallon. As such, 1.38 gallons of propane has the same amount of energy as one gallon of conventional gasoline. https://nhcleancities.org/2017/04/c...oline has,one gallon of conventional gasoline.

 

[Kurtatron]

One interesting thing I remember from thermodynamics.

the hotter an engine runs, the more thermally efficient it could be. If you had a ceramic engine block that could match the temperature of burning gasoline (around 2200F) you would have a much more thermally efficient engine. I think it is because there are less thermal losses between the burning gasoline and engine, if it is running very hot.

 

[KrisZ]

In simple terms, thermal efficiency is the amount of heat put in, vs the amount of heat the system puts out. In a furnace, you want to put out as much heat as being put in and minimize that heat being converted to other types of energy, like mechanical energy.

In a car it’s the opposite, you want as much heat going into the mechanical energy and as little being produced by the engine.


I think it was GM that was playing around with capturing the otherwise lost heat into a boiler and making steam. If I recall, they achieved pretty good fuel economy out of a typical V8 pickup of that era. I think it was 80s.

 

[Rmay635703]

I’m surprised with all the tech no how this site isn’t a hotbed of BSFC chart collection and creation

Brake Specific Fuel Consumption (BSFC) Maps - EcoModder Forum Wiki

ecomodder.com

Lots of efficiency talk here

Nissan introduces engine with 50% thermal efficiency

This is great! I need to watch the whole YouTube video when I get a chance. I love the way the Japanese nod when they are introduced. It's funny though they are dubbed with female voiceovers. 50% thermal efficiency is great, and they have certainly put a lot of effort into this, and it is...

bobistheoilguy.com

Hypermiling works because most cars are aerodynamically terrible and because the engines are way oversized for most speeds meaning they are never running at peak efficiency (BSFC island) most of the time

BSFC values typically align to the thermodynamic efficiency right at peak.

This is why it’s basically never possible to get peak fuel economy with an automatic, a human can still do better rowing gears

We often don't think about efficiency.

Food for thought--why don't we simply run more cars on propane, it's clean?

I didn't know myself that propane has more energy than natural gas.

Ever had a gas furnace not work when it's single digits outside?

The heavier a fuel is the more btus

I live in an area with a fair amount of propane use but it’s extremely difficult to actually fill since legally only authorized… and the hours and # of locations selling at retail is mediocre.

Prices vary from similar to gasoline to rather egregious so the savings isn’t there unless you buy a large tank full in the spring when it’s cheaper, rental fees and delivery fees might kill the deal

 

[kschachn]

One interesting thing I remember from thermodynamics.

the hotter an engine runs, the more thermally efficient it could be. If you had a ceramic engine block that could match the temperature of burning gasoline (around 2200F) you would have a much more thermally efficient engine. I think it is because there are less thermal losses between the burning gasoline and engine, if it is running very hot.

The Japanese did work back in the 90s with adiabatic engines. I don't know if that is still ongoing.