Why are gas turbine engines not used in cars? Is what I think correct?

[k.udhay]

TL;DR Summary

Jet engines Vs ICE - Primary difference is perhaps the pressure during combustion process?

Since about 4 months now, I was constantly thinking how a jet engine works and why it can't replace the ICE for cars, particularly when it has no reciprocating masses. Just two days ago, it triggered me that the power stroke of an ICE makes the biggest difference.

In a diesel ICE, during the powerstroke the the pressure inside the combustion chamber reaches 200 bar (google), whereas the same in a jet engine is just 41 bar (quora)!

This basically means that the torque production of an ICE engine is much higher than that of a jet engine. Simply put, jet engines cannot give the same acceleration as a "comparable-sized" ICE.

Later I did some text book reading to know more about the respective cycles both these engines follow.

Theoretically in both Diesel cycle and Brayton cycle, the pressure is constant thru' out the combustion process - But the actual cycle of diesel just confirms that there is a steep raise in pressure during the combustion process.

This gives me more confidence that my rational on 'why a jet engine cannot outperform on roads' is correct.

This post is mainly to know if my above understanding is correct. Give me your comments pl.!

 

[DaveC426913]

I'm not sure how practical as opposed to academic your interest is in this.

There are myriad factors for propulsion choice that, together, far outstrip mere performance considerations.
Cost, existing technology, parts, maintenance, infrastructure, safety - the list is endless.

Sorry, I realize I am not actually addressing the gist of your post.

 

[gmax137]

https://en.wikipedia.org/wiki/STP-Paxton_Turbocar

 

[jack action]

The main reason why turbines are not used in cars is because although their efficiency can be higher than piston engine efficiency, they cannot maintain it over a wide enough RPM range. Turbines are meant to be used at a constant RPM.

 

[k.udhay]

The main reason why turbines are not used in cars is because although their efficiency can be higher than piston engine efficiency, they cannot maintain it over a wide enough RPM range. Turbines are meant to be used at a constant RPM.

What I also read is that Turbines are not quick for throttle response. Is that true?

 

[Baluncore]

Gas turbines are incompatible with the present start-stop traffic cycle times.

The turbocharger is a partial compromise, that surrounds an ICE, using a piston engine as the final compressor, combustor and the first turbine stage.

The game has changed. Maybe a hybrid EV could apply a small gas turbine with heat exchangers, as a battery charger, but, could it compete with high compression ratio ICEs ?

 

[jack action]

What I also read is that Turbines are not quick for throttle response. Is that true?

With a piston engine, you can increase the fuel to its maximum air-fuel ratio at Wide Open Throttle (WOT) to get the largest pressure in the [closed] combustion chamber.

With a turbine, that has no air restriction, the maximum pressure in the [open] combustion chamber is determined by the outlet total pressure of the compressor. If the combustion pressure is too high the airflow will reverse and this could lead to compressor surge. The following compressor map shows the maximum pressure ratio (the surge line) with respect to its mass flow rate:

So you can only increase the combustion pressure gradually as the turbine speeds up (mass flow rate increasing).

 

[gmax137]

The Indy car I linked to talked about a 3 second throttle lag. OTOH, they nearly won the race. I guess the track is more predictable than city trsffic.

 

[Flyboy]

Your understanding is partially correct, but there’s more to it.

From a purely theoretical standpoint, a suitably designed gas turbine would actually be quite good for long-haul driving such as semi trucks. Yes, they have less torque output when you take the output directly, but you pretty much always gear it down significantly. Look at every gas turbine not used for jet propulsion, i.e. turboprop, turboshaft, marine propulsion, power generation, etc… they all have reduction gearing, often times significantly. Think about how much torque is needed to spin a helicopter rotor system. It’s a lot.

You are correct about the acceleration time required by a gas turbine being much longer. It’s an unavoidable part of the entire concept and is part of why the Navy was initially hesitant about using jets aboard carriers. When you apply full power to wave off and go around and it takes multiple seconds for the engine to reach the power setting, you’re in trouble in those last few seconds before landing. Modern designs are much better about this, but if I go from idle to full power, it’s still at least a three second lag as the engine spools up. On the freeway, probably not a bad thing, but in the city it’s a no-go.

The biggest real-world/practical issues with using a gas turbine are as follows:
-Noise. My god, the noise. You can’t really muffle the exhaust from a gas turbine without having a system nearly the size of a house. It would introduce too much back pressure and degrade performance. So, you have to live with the whistle/scream of the turbine exhaust.

-Cost. A gas turbine is a very finely and precisely designed machine, and requires extremely tight tolerances the smaller you get. That means that your manufacturing process is more complex and expensive, and your maintenance requires specialized skills and equipment.

-Scaling. A piston engine is surprisingly tolerant of looser tolerances due to the nature of the design, and so scales down significantly better than a turbine.

-Fouling. They did try using gas turbines on trains but if memory serves, one of the biggest issues was the sheer amount of dirt/dust that built up on the compressor blades, steeply cutting into efficiency. Planes and helicopters don’t deal with constantly being low and slow at ground level, with all the dust, or if they do, they have defined limitations on their operational time in those conditions. Proper filtration should fix the issue, but that could be costly and bulky.


@Baluncore hit on a good compromise: hybrid drivetrain. Use the gas turbine to spin a generator at its optimum rpm, then use the output of the generator to drive the wheels. Trains use a similar system already to great effect, albeit with diesel engines. I would argue that pairing it with batteries would be ideal for time spent in the city. Less noise, much better response time. Then switch over to the turbine by driver command for longer drives on the freeway. But if you’re going that route, might as well run a turbo diesel as your powerplant. It would be easier to make that transition from current designs.

 

[gmax137]

-Noise. My god, the noise. You can’t really muffle the exhaust from a gas turbine without having a system nearly the size of a house. It would introduce too much back pressure and degrade performance. So, you have to live with the whistle/scream of the turbine exhaust.

And how. There was a guy in town who ran a jet car at the local drag strip. When he fired that thing up outside his shop (mid-week tuning?) everyone knew it.

 

[k.udhay]

Your understanding is partially correct, but there’s more to it.

From a purely theoretical standpoint, a suitably designed gas turbine would actually be quite good for long-haul driving such as semi trucks. Yes, they have less torque output when you take the output directly, but you pretty much always gear it down significantly. Look at every gas turbine not used for jet propulsion, i.e. turboprop, turboshaft, marine propulsion, power generation, etc… they all have reduction gearing, often times significantly. Think about how much torque is needed to spin a helicopter rotor system. It’s a lot.

You are correct about the acceleration time required by a gas turbine being much longer. It’s an unavoidable part of the entire concept and is part of why the Navy was initially hesitant about using jets aboard carriers. When you apply full power to wave off and go around and it takes multiple seconds for the engine to reach the power setting, you’re in trouble in those last few seconds before landing. Modern designs are much better about this, but if I go from idle to full power, it’s still at least a three second lag as the engine spools up. On the freeway, probably not a bad thing, but in the city it’s a no-go.

The biggest real-world/practical issues with using a gas turbine are as follows:
-Noise. My god, the noise. You can’t really muffle the exhaust from a gas turbine without having a system nearly the size of a house. It would introduce too much back pressure and degrade performance. So, you have to live with the whistle/scream of the turbine exhaust.

-Cost. A gas turbine is a very finely and precisely designed machine, and requires extremely tight tolerances the smaller you get. That means that your manufacturing process is more complex and expensive, and your maintenance requires specialized skills and equipment.

-Scaling. A piston engine is surprisingly tolerant of looser tolerances due to the nature of the design, and so scales down significantly better than a turbine.

-Fouling. They did try using gas turbines on trains but if memory serves, one of the biggest issues was the sheer amount of dirt/dust that built up on the compressor blades, steeply cutting into efficiency. Planes and helicopters don’t deal with constantly being low and slow at ground level, with all the dust, or if they do, they have defined limitations on their operational time in those conditions. Proper filtration should fix the issue, but that could be costly and bulky.


@Baluncore hit on a good compromise: hybrid drivetrain. Use the gas turbine to spin a generator at its optimum rpm, then use the output of the generator to drive the wheels. Trains use a similar system already to great effect, albeit with diesel engines. I would argue that pairing it with batteries would be ideal for time spent in the city. Less noise, much better response time. Then switch over to the turbine by driver command for longer drives on the freeway. But if you’re going that route, might as well run a turbo diesel as your powerplant. It would be easier to make that transition from current designs.

Thank you so much for such a detailed answer. I am fully clear with the answers!

 

[Dullard]

an M1 Abrams tank does use such a powerplant. Some were delivered (over US objections) to Ukraine. They were apparently not a big hit for some of the reasons described above.

 

[Vanadium 50]

I don't think that the problems that Ukraine experienced are strongly related to the GT engines. More that they are 40 years old, and they are fielding a small number of them. Maintenance and spares are tougher if you have a dozen of something than a zillion.

That said, for any technology change, one should ask "what problem are we trying to solve?" The answer in this case seems to be "automotive fuel efficiency" where there seem to be much cheaper ways to accomplish the same thing.

 

[Flyboy]

I don't think that the problems that Ukraine experienced are strongly related to the GT engines. More that they are 40 years old, and they are fielding a small number of them.

That, and there aren't as many situations on the battlefield where they can operate in the manner they were designed to: fighting in large numbers against an active, massed invasion of combined forces that are of inferior quality. The fight in Ukraine is a very different kind of warfare and the Abrams are struggling just as much as the Soviet T-series tanks.

Buuuuut, that's a conversation for a different forum altogether, isn't it?

And how. There was a guy in town who ran a jet car at the local drag strip. When he fired that thing up outside his shop (mid-week tuning?) everyone knew it.

Well, that's a case of opting for sheer thrust production, not being compliant with noise controls. The noise in that case is both the noise of the engine itself and the interaction of the high subsonic to low supersonic exhaust plume and the still air around it. Rockets have the same thing and it's part of what gives that distinctive crackle.

I'm not entirely sure what part the actual engine itself has on the noise of the exhaust, though. Not something we covered in school for mechanics.

But, things like forced exhaust mixers and chevrons on the trailing edges of the exhaust shrouds on civilian engines are intended to make the transition between air and exhaust streams happen in a more controlled manner, and I think it raises the frequency of the noise, making it less likely to travel long distances. Having worked with two different versions of the Pratt & Whitney JT15D engine, one with the core flowing through a circular exhaust and one flowing through a forced mixer, it makes a remarkable difference. The engine with the mixer was undeniably quieter at takeoff power, whereas the circular exhaust sounded like a small fighter jet.

 

[cjl]

In addition to Flyboy's excellent answer, the final part of it is that gas turbines have atrocious idle and part throttle fuel consumption. They're actually quite efficient at high power, but the amount of fuel they burn just to stay running at low power levels is much, much more than what is required to keep a piston engine idling. Since cars spend a fairly significant amount of their time idling (at stop lights, in traffic, etc), this means that despite the high efficiency at high power levels, you end up burning much, much more fuel in a turbine car than in a piston car due to this terrible part throttle and low throttle efficiency.

 

[Vanadium 50]

If you get an app like Torque for your car, you will be shocked at how little time you spend at high power.

 

[russ_watters]

If you get an app like Torque for your car, you will be shocked at how little time you spend at high power.

If you're an American. I was in Italy last month and rented a "city car", and basically every acceleration was full power. Before I even got to the alps.

Cars like that don't exist in the US.

 

[Vanadium 50]

There is the Honda Fit. But I get your point.

In Europe, where you have twice the incentive for good fuel economy, they have not adopted automotive GT engines. That tells you something right there.

PS I drove a Peugeot 103 for a while.

 

[gmax137]

I drove a Peugeot 103 for a while.

I had to look that up. I forgot about their mopeds. Peugeot cars are pretty rare in the US now. When I hear the name I think of these. We called them "pig outs" when I was a kid. I guess they were actually pretty nice cars.

 

[russ_watters]

There is the Honda Fit. But I get your point.

...Peugeot 103

Just to close the loop, the car I rented was a Lancia Ypsilon hybrid. If I've got the right variant, it's a 1.0 L i3, tearing at the pavement with 69 horses. Google tells me the base Fit - a very similar looking car - is 1.3 L, 99 hp. And lighter.

Last time I'd driven in Europe was 2013. I don't remember the car, but I'm sure I'd remember if it was that under-powered.