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Diesel: why is it not taking off?

cooling has been solved by “throwing more fuel through the engine” and people have accepted it as a optimal engineering design in spite of having no real idea (until the advent of GPS) what power settings and speeds they fly at for a given fuel flow.

However, that has not been true for any pilot with a brain and who can read, for about 10 years. Peak EGT or LOP operation is well established, despite some diehards remaining – like those Japanese soldiers who thought the war carried on for another 20 years

How is a design that is heavier AND uses 4x more fuel for the same amount of output power

4x more fuel? What engine is that? The Lyco/Conti engines achieve an SFC which is state of the art for a constant power output application. They are at least as good as any petrol car engine. They do have serious issues with thermal management but again any pilot with a brain can deal with it, but it is a consequence of light weight (air cooled, thin metal sections) and control system simplicity (fixed ignition timing, etc) which are not trivial to address while maintaining a fairly high reliability and easy servicing in the field where expertise is, ahem, in a short supply…

Not in a flying school environment. An o320 last 2400 hours. Plus u can bet on a new top end at half life. Plus numerous days lost due mag problems and spark plug fouling.

That, however, is due to dumb operating methods, absolute bottom-end-quality “maintenance” practices (IME) and occasional gross abuse (Vx climbs all the way to 2000ft, PFLs, no [working] engine instrumentation so CHT remains unknown as it creeps past 500F, etc) in the school environment. In my PPL, the red lever was never used. So, you are (a) burning some 30% more fuel and (b) fouling up not just the plugs but the whole engine. It is done to keep the PPL simple. Let’s face it, some 90% of PPL holders chuck it away more or less right away, and it is widely assumed they wouldn’t pay for a better course (which manifests itself in other areas e.g. the general reluctance by schools to teach GPS usage). The downside of that is that the occasional more motivated customer isn’t taught how to fly a plane properly from A to B and learns almost everything of relevance from internet forums…

As soon as the thirleirt gets a 1800 hour TBR from a flying school prospective the lyco will be dead in the water.

I think we would all like to retrofit a diesel, for all the obvious Europe-specific reasons, but nobody who brings out a decent retrofit option is likely to price it competitively. It will always be priced at the level of a high usage customer (a few hundred hours a year) where the financial case hangs on a knife edge – like running a small jeweller’s shop on the high street where you have to flog 1000 quid’s worth of gold every day just to pay the rent and the business rates.

I don’t think most schools have the capital.

If there was a direct IO540 replacement for under say €50k, and it was proven-reliable, I would install it tomorrow.

Last Edited by Peter at 15 Jun 10:09
Shoreham EGKA, United Kingdom

Not trying to defend Lycoming or Continental, I wish there was a better engine option for my plane… (Thielert doesn’t fit, it’s too big)

Shorrick Mk2 what car are you driving? It seems to be either massively underpowered, you’re driving way above legal speeds, or you are driving constantly uphill.

Yes, the “car cruises at 25%” mantra is wrong – it’s actually less.

Do the math on yours – mine cruises at 65% power (@2.5 gph…)

Care to elaborate?


For my car, cw*A=0.69 (taken from an advertising brochure of the car manufacturer). rho=1.293kg/m^3. Assume 100km/h unaccelerated cruise (v=27.7m/s). So P=9561W, which is about 13BHP. My car is rated 160BHP, so about 8% of the engine power is used to overcome drag when cruising at 100km/h. There are additional losses (friction), so actual engine power is somewhat bigger than that.

How is a design that is heavier AND uses 4x more fuel

Again, care to explain?

Lycoming specifies for my engine (IO360) 9.5USG/h for 65% economy cruise (130BHP). In my experience, theses numbers are correct. So this is 57lb/h fuel consumption, or a BSFC of 0.43lb/BHP/h.

My car (or even most cars) have around 200g/kWh, which is about 0.33lb/BHP/h.

The best marine two stroke diesels seem to be at around 0.25lb/BHP/h.

While surely a remarkable fuel saving, it’s less than a factor of two for the marine diesel, or about 30% improvement for the car engines, and thus nowhere near a factor of 4 improvement.

But don’t let facts interfere with a nice “discussion”

LSZK, Switzerland

However, that has not been true for any pilot with a brain and who can read, for about 10 years. Peak EGT or LOP operation is well established,

You seem to assume that the typical piston GA flight is a 7h leg from EGKA to LDLO. I assure you it’s not. The vast majority of flights are traffic patterns and short flights of which a large part (majority?) you spend in full rich climb. Piston GA airplanes are not transport devices, they are recreational and training devices for the vast majority. I flew my C172 with 28l/h average (because my legs were ca 2h in average) but for the charter customs, I had to calculate with around 33l/h and it’s not because none of them was “with a brain and who can read”.

An engine that has a good SFC in all stages of flight and that cannot be easily destructed by a pilot incorrectly operating the levers is a great thing. The potential 1800h Thielert will find its market, I’m sure about that. Let’s hope Continental can also make it financially more attractive.

Yes, let’s stick to facts. An immense asset of this forum that we usually seem good at that.
LeSving, fyi, I don’t know who told you that an overhaul of an Austro is twice the cost of a new Lycoming. It’s around half (15keuro).
Austro is working on getting TBR to 2400 and Conti to 1800 (135HP version).

Private field, Mallorca, Spain

You seem to assume that the typical piston GA flight is a 7h leg from EGKA to LDLO

Absolutely not! Anyway that leg is more like 5hrs unless you have a huge headwind. Also such long flights are very rare. Nevertheless, the capability represents a lot of value which in turn drives the market for aircraft thus capable.

My average leg is probably 1.5 to 2hrs, but the fuel burnt in the climb is small. I count on 6-7 USG for the climb to FL120, and one will travel some 50-70nm doing that anyway. The climb is done using a constant EGT method so not “full rich”.

And, as I’ve posted above, the flying school scenario – which does account for a large % of GA mileage – involves a very inefficient operating profile. For them, a FADEC solution is worth paying for – but for the wrong reasons. In fact a plane which is electric and lasts just 1.5hrs would probably be quite good for a lot of PPL training

Also almost no private owner-pilot or syndicate who flies just short burger runs is going to pay for any diesel retrofit. The value simply isn’t there, in that type of mission profile. Even if a diesel retrofit was 30k there would be almost no market, because the only time you could justify it would be at overhaul time, and most owners never get anywhere near TBO on that mission profile. Given that flying abroad represents so much value in GA, it is almost a given that the burger run market doesn’t have the money to spend on a conversion. I recall a few conversions in the UK over the years, and the general verdict seems to be that it was an “interesting experiment” but they wouldn’t do it again.

That leaves just a small % of owners (or syndicates) who do reasonable legs, and that’s probably why the diesel retrofits we have seen to date are priced for the flying school market! The vendors are not stupid; they know the market. “We” may disagree with it but that’s a bit like wanting a 5 quid landing fee at Gatwick.

an overhaul of an Austro is twice the cost of a new Lycoming. It’s around half (15keuro).

Depends on which Lycoming. Last time I looked (a few years ago) an IO540-C4 was about $65k, from an outfit which was selling them cost-plus which Lyco hated. But almost nobody buys a new Lyco unless theirs is totally smashed up or something like that.

Last Edited by Peter at 15 Jun 11:10
Shoreham EGKA, United Kingdom

Last time I looked (a few years ago) an IO540-C4 was about $65k

Vans sell factory new certified Lycomings for:
IO-540 260HP (RV-10 only)
Part Number = EA IO-540-D4A5 RT
Price = $50105.00

and the experimental version (Also directly from Lycoming)
Experimental IO-540 260HP (RV-10 only)
Part Number = EA XIO-540-D4A5 RT
Price = $46600.00

But of course you have to purchase an RV-10 kit with it

The Austro/Thielert is more like a 320/360 (hardly but still), and those engines is around 30k$. ACS sells them for 25k (Superior XP), which is hardly more than a Rotax 912 ULS.

The elephant is the circulation

I drive a Volvo S60. Cruise speed is 120kmh, torque at cruise rpm 315 lb/ft, rpm at cruise 2525. I let you derive the power output, but if my maths are right it’s about 65% of 260 hp top power. At that speed and power setting it uses about 9.6 lph (with the aircon on…).

it’s about 65% of 260 hp top power … it uses about 9.6 lph

Yeah right. This would work out at a BSFC of 0.09lb/BHP/h. A BSFC of 0.14lb/BHP/h would mean an energy efficiency of 100%. So you claim an energy efficiency of 155%. Seems your car is a Perpetuum Mobile

LSZK, Switzerland

The numbers I found for your car: cw=0.29, width 1.865m, height 1.484m, so that works out at cw*A=0.8

So P=19156W or about 26BHP. 10% of rated BHP for drag. If you needed another 55% for friction and your aircon, then something’d be seriously wrong with your car.

LSZK, Switzerland

I think the average 2 litre petrol car, top speed ~130mph, is running at about 20% of max power when doing the UK national speed limit of 70mph.

I also doubt the engine is running at peak EGT (best economy) at 130mph. It would be pointless to do so, given that nobody cares about the MPG at 130mph.

Shoreham EGKA, United Kingdom
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