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.