8 replies to this topic

[Wuzak]

Just finished reading an article on the Jaguar C-X75 hybrid supercar (EVO, December 2010).

The C-X75 is driven by 4 195hp electric motors, each of 50kg, driving the wheels via drive shafts. Energy is stored in a batery pack in the spine of the car. Range on batteries alone is 60 or 70 miles.

Range extending is achieved by use of a pair of microturbines, each connected to its own generator.

The turbines have been developed by Jagar Land Rover in partnership with Bladon Jets, with a UK government grant.

So far the car has only operated purely on electric power, the turbines and generating systems have yet to be


Each generator/turbine unit is claimed to weigh 35kg, of which the axial flow turbine contributes 3kg while producing 94bhp. Turbine operating speed is said to be 80,000rpm.

The author says:

when both turbines on the C-X75 are spinning at 80,000rpm and producing maximum power, they consume a total of 70,000 litres of air per minute. Compare that with the 7500 litres of air needed by a 5-litre engine producing its maximum power at 6000rpm and you can see how greedy those turbines are.

Which doesn't seem right to me...So I ran some numbers.

If the turbines ran an air:fuel ratio of 15:1 my numbers suggest an efficiency of about 3%. If the efficiency was in fact 30% and power remained at 94hp, the air:fuel ratio would be over 140:1. If the AFR is 15:1, and the efficiency is 30% then my calcs show the air required is just under 7350l/min, or 3675l/min for each turbine.

Either there was a typo in the article, some misinformation was fed to the journo, or my numbers are out. Just seems odd to me that two turbines making a combined 200hp should use 10 times the air of a 5l engine at 6000rpm, which would probably be making 300hp+!

Most microturbines to this date have been of the centrifugal compressor/radial turbine type, basically using turbochager parts. This one uses an axial flow compressor and turbine, which, in theory, should give better efficiencies. IIRC most microturbines of the simpler type have peak efficiencies of around 25%.

A couple of questions: How much power would the HP turbine driving the compresor be making? How does the flow rate of the gas through the 94hp power turbine compare with an engine that will be used in 2013 for F1?

[Grumbles]

Could it be that only a portion of that air passes through the combustion chamber? I don't know much about gas turbines but I think it's not uncommon to have some of the air bypass the combustion chamber and recombine with the exhaust downstream of the chamber. That way it is still used as part of the working fluid but "dilutes" the hot exhaust gas and limits the temperature that the turbine sees.
Having said that the numbers still don't seem quite right..

[Wuzak]

That would be a turbofan. The thrust from a turbofan on a jetliner comes primarily from the fan section. But for the microturbine there may be a small amount of bypass air to help cool the combustion chambers and kep them from melting, but it would only be a small fraction of what goes through the combustion chambers.

[Grumbles]

I was thinking more along the lines of a stationary industrial type gas turbine. But I always thought the bypassed proportion to be fairly small, as you say.

[gruntguru]

I was thinking more along the lines of a stationary industrial type gas turbine. But I always thought the bypassed proportion to be fairly small, as you say.

I have heard up to 300% excess air (not turbo-fan either) but that is still way short of the 900% in the Jaguar - assuming Wuzak's calcs are correct.

EDIT. Just did a quick check and get 8,500 l/min at 30% efficiency so 70,000 l/m is about 725% excess air. That's a lot of bypass.

Edited by gruntguru, 27 December 2010 - 08:06.

[Catalina Park]

That's a lot of bypass.

Or a lot of hot air.

[SteveCanyon]

Or a lot of hot air.

Indeed.
The smaller the jet engine the (typically) less efficient they are. Viscosity of the air and so on.

[Kelpiecross]

Just finished reading an article on the Jaguar C-X75 hybrid supercar (EVO, December 2010).

The C-X75 is driven by 4 195hp electric motors, each of 50kg, driving the wheels via drive shafts. Energy is stored in a batery pack in the spine of the car. Range on batteries alone is 60 or 70 miles.

Range extending is achieved by use of a pair of microturbines, each connected to its own generator.

The turbines have been developed by Jagar Land Rover in partnership with Bladon Jets, with a UK government grant.

So far the car has only operated purely on electric power, the turbines and generating systems have yet to be


Each generator/turbine unit is claimed to weigh 35kg, of which the axial flow turbine contributes 3kg while producing 94bhp. Turbine operating speed is said to be 80,000rpm.

The author says:



Which doesn't seem right to me...So I ran some numbers.

If the turbines ran an air:fuel ratio of 15:1 my numbers suggest an efficiency of about 3%. If the efficiency was in fact 30% and power remained at 94hp, the air:fuel ratio would be over 140:1. If the AFR is 15:1, and the efficiency is 30% then my calcs show the air required is just under 7350l/min, or 3675l/min for each turbine.

Either there was a typo in the article, some misinformation was fed to the journo, or my numbers are out. Just seems odd to me that two turbines making a combined 200hp should use 10 times the air of a 5l engine at 6000rpm, which would probably be making 300hp+!

Most microturbines to this date have been of the centrifugal compressor/radial turbine type, basically using turbochager parts. This one uses an axial flow compressor and turbine, which, in theory, should give better efficiencies. IIRC most microturbines of the simpler type have peak efficiencies of around 25%.

A couple of questions: How much power would the HP turbine driving the compresor be making? How does the flow rate of the gas through the 94hp power turbine compare with an engine that will be used in 2013 for F1?

From memory, - a jet engine has about a 70:1 air to fuel ratio. The A/F ratio can't get much less than this or the exhaust temperature would become too high and damage the turbine blades. The excess air is used, in a sense, as cooling for the turbine.

[J. Edlund]

As an example, a Rolls Royce Avon Mk 60 turbojet consume about 1.5 kg fuel per second at full power, the compressor supplies 78 kg/s, so there is more than 50 kg air per kg fuel. Infact, only part of the airflow is supplied to the combustion chambers in the engine. Most of the air go around the combustion chambers in order to cool them while only a small part of the air enter in front of the flame holder, used to supply the combustion with oxygen. But all of the air will go through the turbines (which is not the case with a turbofan).

The efficiency of a small gas turbine isn't that great either. For instance a Garrett JSF100, a small 37 kg 90 hp gas turbine (one stage centrifugal compressor, one stage axial turbine and one stage axial power turbine) consume 800 g/kWh. That is roughly an efficiency of 10%. An Allison 250, a 62 kg 317 hp helicopter engine is better, but it still consumes 430 g/kWh, an efficiency of slightly below 20%.