Efficiency Question

[MikeSamuel]

Hello...

...I've just been pondering the efficiency of small aircraft engines in relation to a normal car engine. Say for a 172 cruising with 4 up, how does this relate in miles per gallon of fuel? Would welcome any comments on this and other efficiency issues...It's not important, I'm just curious :D

Regards,
MS

[tomcs]

Hi! I fly the PA-28 and it does about 32 litres and hour. Which is around 7 Imp Gal an hour. Take a cruising speed of about 100 knots IAS. This works out at about 14 nm to the gallon or about 3 nm to the litre. Basically its a lot less than the efficiency of a family car. However u have to realise that these aircraft engines are huge in relation to the average family car. Even though they are still four cylinder they are between 5 or 6 litre engines as opposed to 1.5 to 2!! So u are going to get much higher fuel consumption even though the max revs. are less. Hope this helps

Tom

[Genghis the Engineer]

Getting on my hobby horse for a moment, you should also bear in mind that the technology behind a Lycontinental is about 60 years old - so it's hardly surprising that they are much less efficient.

Let's say perhaps a C150, burns about 5 gal/hr at about 100 mph - or about 20 mpg.


There are more modern engines, a good example being the Rotax 912, which at 85hp is similar to a middling Continental. On a Pegasus CT this gives a 110kn (130mph) cruise at about 15 litres/hr. This comes out at about 38 mpg, or around the same as a medium sized petrol engined car.

G

[MikeSamuel]

Thanks for your excellent replies guys...I thought that the age of the technology would explain it in some way. So if these engines are maybe 5 litres, is the fact that they are producing around 100 hp just due to this lack of engineering development? Or is something lost in the translation between driving wheels and driving a prop?
Would one engine driving two props be out of the question?

Thanks,

MS :D

[twistedenginestarter]

Let's take tomcs's figure of 16 mpg. First off you have got to realise this is at 120 mph. My car would struggle to better that at that speed. (in fact I would have to be travelling whenever possible at my max speed of 144mph in order to get anywhere near the aircraft's leg times). Secondly flying is shorter than driving. Taking Leicester to Manchester - it is roughly 70nm by air but 120m by car. Flying is therefore surprisingly efficient.

Until of course, all the CAA nonsense gets factored in. Then it is a quaint passtime for minor toffs with more money than sense.

[twistedenginestarter]

So if these engines are maybe 5 litres, is the fact that they are producing around 100 hp just due to this lack of engineering development?

The fact that they have low bhp per litre does not mean they are particularly inefficient. Modern cars are more efficient than they used to be but a lot of that is to do with aerodynamics and weight reduction rather than the engines. I doubt modern car engines are fantastically more fuel efficient than they were 30 years ago.

A key factor in efficiency is compression ratio. One reason why diesels are the way forward.

[tomcs]

Hey guys!! Please dont get me wrong!! I think the Lycoming is an excellent engine. I think the reason is that they want it to be reliable as possible and as the saying goes....why mend something if it's not broken! And I would rather go by plane anyday...than car! And as you say if u did it in terms of time efficiency the a/c would win HANDS DOWN :)

Tom

[M.Mouse]

twistedenginestarter

I have to disagree.

Let me see we have modern fuel injection systems (and variations of), variable valve lift & timing, more valves per cylinder, higher operating temperatures, computerised engine control, turbochargers and superchargers, high compression ratios compared to say my side valve Ford 105E engine with 8 pushrod valves and a single carburettor built with heavy con-rods and pistons

I think you might find that while aerodynamics and weight play a part it is plainly wrong to make the statement that you made.

[tomcs]

would one engine driving two props be out of the question

I've only ever known that to be the case on turboprops. eg the Antonov AN-70. I'm not sure about the ins and outs perhaps thats for Genghis to say!

And they also have weird shaped props on those a/c

Tom

[Tinstaafl]

The engines, like all things in aviation, are subject to a number of compromises.

The airframe design dictates how much HP is needed for the intended performance. Another way of thinking about it is that engine power available dictates what sort of airframe & performance is possible.

Taking a C172 as an example, the designer went for a combination of design trade offs that resulted in an a/c that cruises at approx 110 kts, can hold 4 bums or 189 litres of fuel, some bags, stalls somewhere in the high 40 kt region, produces a certain amount of drag etc etc etc.

This required 150 or 160 hp.

To produce this HP the engine designer has lots of options, all with penalties. Some things to be considered:

Number of cylinders:
more cylinders = more weight, friction losses, mechanical parts, size

Compression ratio
Significantly affected by fuel type

Capacity
More capacity=more HP.

Normally aspirated vs forced induction
Forced ups the power but has fuel & equipment demands

Induction system eg number of valves
More valves can improve efficiency but at the cost of complexity & potential reliability

Fuel metering eg carby vs injected
Cost, complexity

Ignition system
Magnetos are self sufficient. If they're turning they're sparking. Even switching them off fails to operational eg the grounding wire coming adrift.

A battery/coil system must have the battery. Lose the battery = lose ignition, ditto the condenser. What about reduncy in the system? That would mean a second battery, coil, distributor etc leading more weight.

Operating RPM limits
Prop RPM is *very* limiting, due to large prop efficiency losses when the tips approach M1.0. This in turn limits the engine RPM. Higher engine RPM is possible using a reduction gear box but this adds more weight & complexity.

Number of cylinders/capacity also effects RPM. Very large pistons don't cope with high RPMs.

Cost of manufacture.
Simple is usually good. As is proven systems.

Redundancy of certain systems
Dual ignition, alternate induction air

Cooling
Air cooled is simple but sensitive to power/air flow/drag issues.
Liquid cooling is more efficient but gives another system to fail.

Space
Smaller is better for drag reduction

...and so it goes on.

All other factors being equal, the manufacturer has a simple choice of high RPM/low capacity or low RPM/high capacity.

A low capacity, high revving engine will need a reduction gear box, adding weight & complexity.

A low revving engine can be connected directly to the prop but will need a relatively high capacity.

BTW, diesels are a similar capacity/RPM trade off.

As for efficiency, aero piston engines aren't doing too badly. A C172 cruises at 110kts/126 mph on ~7 imp gal ph.

That's 18 mpg.

What sort of economy do you think your average 4 seater car gets at that speed?

[simon brown]

Interesting question and one thats somewhat difficult to quantify.

Not wishing to be pedantic but MPG figures are irrelevant in avaition. Aircraft fuel consumption is quoted in galls/litres per hour
and that varies under the prevailing conditions etc

I dont know what manufacturers use as their yardstick for the fuel burn rate. Standard Asmospheric conditions, 20deg c zero wind, new aircraft etc etc. We have one Warrior in our club that could drink Oliver Reed under the table and its relatively new!!....

If you hover a helicopter for an hour without covering any distance you are doing zero miles per gallon but still using fuel as you would in your car in a stationary traffic jam. So there are differences here

sb

[englishal]

Although that nice diesel (Jet A1) PA28 in flyer a couple of months ago seems to be the way forward. Cruised at around 90 kts for the cost of 6 quid an hour.

Bargain...
EA

[Tinstaafl]

Actually, using MPG is not irrelevent. Ever heard of Specific Air Range, Specific Ground Range, Air or Ground Nautical Miles per US Gal/Imp. Gal/Kilo/Pound/Litre? Fuel consumption is not always quoted in a quantity per hour nor is it always in Gal/Litres.

Using MPG for the a/c is a simple conversion of quantities to give a common reference. I could have converted my car's fuel economy into a quantity per hour if I wished.

Ground transport fuel economy is also affected by headwind/tailwind. Since this is a variable that affects both ground & air the - admittedly unstated - presumption is that this was comparing equivalent, like-for-like conditions ie nil wind.

I also chose to use one of the commonest cruise conditions - a fairly typical 65-70% power setting to compare efficiency as a function of speed. Had I wished I could have chosen an economy cruise of ~45-50% & greatly improved the demonstrated economy of the a/c.

Another exercise would be to compare economy for both types of vehicles each at their 'normal' cruising speed eg the C172 as stated in my earlier post, and a 4 seat car with similar capacity on the motorway doing the legal limit & in top gear.

Similarly one could also compare vehicles each at its own best economy speed.

You could also derive an index of economy that includes the load or load as a proportion of Empty weight as another variable.

What term is used to compare efficiency is simply a case of convenience. Measuring the fuel ecomomy of both types of transport involves the variables Fuel Consumed (in whatever units you like), Time, Speed & Distance. It's only a matter of converting units.

The hovering helicopter example is spurious. A car can also have its engine running but with the gearbox in neutral. Similarly any vehicle can be operated in a way that minimises economy - but that wasn't what was asked.

[twistedenginestarter]

Let me see we have modern fuel injection systems (and variations of), variable valve lift & timing, more valves per cylinder, higher operating temperatures, computerised engine control, turbochargers and superchargers, high compression ratios compared to say my side valve Ford 105E engine with 8 pushrod valves and a single carburettor built with heavy con-rods and pistons

You're confusing efficiency with other benefits.

Turbo/Superchargers provide more bhp for a given amount of engine displacement/weight/size rather than fuel consumed, and anyway were on cars when your grandad was a twinkle in the milkman's eyes.

Similarly variable valves are for wide rpm range efficiency - not necessarily having a major impact on steady cruise consumption. More valves increase volumetric efficiency (=brake mean effective pressure) which helps efficiency but has a downside in temperature and stress. Aircraft engines avoid these for reliability reasons.

I agree engines have got a bit more fuel efficient (maybe 20-30%) but Lycoming and Continental choose huge engine size and low revs not soley because they massively behind today's technology but more because this is thought to be safest for the operating conditions of aircraft.

You have to ask yourself what priorities you would most like in your C172 half way between Liverpool and Ronaldsway on a dark January night with you and your family on board...

[expedite_climb]

Twisted -

Not sure it is quicker by air unless you are looking at v long distances.

The old phrase 'Time to spare, go by air'

you have to :

i) Get to the airport
ii) Get the MET
iii) Plan the trip
iv) Get out to a/c
v) Preflight the a/c
vi) Fuel the a/c
vii) All your start checks and taxiing.

E.g over 200 miles you are looking at flight time of at least 2 hrs. Thats only and hour shorter than the car (okay i drive to fast and it depends on roads etc..)

All depends on lots of things really....

[basil fawlty]

Unfortunately, we are not comparing like with like here. They are designed with different considerations in mind. As stated previously there are too many differing factors between air travel and road travel. The only way to know for sure is to put say a 100 hp aero engine and a 100 hp motor car engine in to a test cell, driving identical loads under identical conditions. Put 10 gallons of fuel in each tank and see which one runs out first.

[MikeSamuel]

Thanks again for all your replies, as I said before I was simply curious to have an estimate of efficiency, and there are obviously different factors which affect this in each case.
It seems that there is little to choose between the two when it comes down to it, except of course that flying is soooo much better ;)
Interestingly, did anyone read the article on the microlight in this month's Pilot? Very desirable, and efficient too!