Jet A vs gasoline

[genna]

I need some education. The diesel thread got me thinking.

Avgas is problematic. It's bad for environment and expensive. More importantly, it's mostly not available outside of North America. The solution has been to develop diesel aircraft engines to operate on jet fuel. I understand that it's been difficult and costly.

What I don't quite understand is why diesel and not your average unleaded gasoline, with or without ethanol. This stuff is also readily available around the world and works pretty well in designed-for-it piston engines of all kinds.

I understand that automotive engines are not the same as aircraft. But if you are going to design(or adopt) a brand new engine(diesel to run on jet A), why not do it for gasoline? I cannot believe that it would be that difficult to adopt an existing unleaded automotive engine for aircraft use by a major manufacturer(Honda?).

Is it just a function of what is already available at the airports?

 

[bnt83]

I need some education. The diesel thread got me thinking.

Avgas is problematic. It's bad for environment and expensive. More importantly, it's mostly not available outside of North America. The solution has been to develop diesel aircraft engines to operate on jet fuel. I understand that it's been difficult and costly.

What I don't quite understand is why diesel and not your average unleaded gasoline, with or without ethanol. This stuff is also readily available around the world and works pretty well in designed-for-it piston engines of all kinds.

I understand that automotive engines are not the same as aircraft. But if you are going to design(or adopt) a brand new engine(diesel to run on jet A), why not do it for gasoline? I cannot believe that it would be that difficult to adopt an existing unleaded automotive engine for aircraft use by a major manufacturer(Honda?).

Is it just a function of what is already available at the airports?

Diesel development is more attractive for worldwide sales for just the reasons you stated, availability of jet fuel.

Automotive gasoline has been used in lower compression piston aircraft for decades, unfortunately its increasing more difficult to find non-tainted (ethanol-free) gas.

 

[genna]

Diesel development is more attractive for worldwide sales for just the reasons you stated, availability of jet fuel.

Automotive gasoline has been used in lower compression piston aircraft for decades, unfortunately its increasing more difficult to find non-tainted (ethanol-free) gas.

I get that MoGas has been used on certain current engines. I also get that you can't use ethanol gas in current aircraft engines. But this is not what i'm talking about. I'm specifically referring to designing new engines for different fuel. It shouldn't be that difficult to get same level of power from 93 octane with a more modern engine design. And probably more reliability to boot. Ethanol or not.

Given that gasoline can be used for other things, it may be a lot less difficult to convince airport operators to install a tank for it.

 

[Capt. Geoffrey Thorpe]

I also get that you can't use ethanol gas in current aircraft engines.

Some current engines. Others, snot a big deal.
I assume that most of what I run in my Rotax 912 is E-10, but I don't even bother to check.

I suspect that, currently, there are a lot of airports that offer Jet A and nothing else.

 

[midwestpa24]

I get that MoGas has been used on certain current engines. I also get that you can't use ethanol gas in current aircraft engines. But this is not what i'm talking about. I'm specifically referring to designing new engines for different fuel. It shouldn't be that difficult to get same level of power from 93 octane with a more modern engine design. And probably more reliability to boot. Ethanol or not.

Given that gasoline can be used for other things, it may be a lot less difficult to convince airport operators to install a tank for it.

Part of the problem is making whatever new fuel you select compatible with the existing fleet. That is where the FAA has been researching a drop in 100LL replacement. We need a cost effective solution for the entire fleet. The reason for the interest in diesel technology is that it will run on existing Jet-A.

 

[chemgeek]

Automobile engines and aircraft engines operate in completely different power regimes. An aircraft engine runs at a large fraction of full rated power nearly continuously, whereas automotive engines operate at relatively low power in highway cruise. Auto engines will develop full rated power only at much higher rpm than aircraft engines, typically requiring gearing (additional complexity and weight) to turn the prop at subsonic speeds. These are some of the engineering challenges. Aircraft engines are actually well-designed for what they have to do: high power at relatively low rpm. What would make them more efficient is electronic timing, ignition, and mixture control. This is more or less where the various diesel engine design variants have gone.

On the fuel side, aviation gasoline must meet different and more stringent standards for vapor pressure, etc. than automotive fuel for flight safety. In addition, Jet-A can be made in more abundance from crude petroleum than gasoline. Gasoline is a rather choice distillate from petroleum refinement. Jet-A is a more readily available fuel than gasoline many places.

 

[ElPaso Pilot]

^^^ This.

And the infrastructure for Jet A is already in place at the vast majority airports around the globe.

Putting in new tanks and distribution for another fuel would not be inexpensive. Many (most?) GA airports are not exactly flush with available cash flow.

 

[genna]

Automobile engines and aircraft engines operate in completely different power regimes. An aircraft engine runs at a large fraction of full rated power nearly continuously, whereas automotive engines operate at relatively low power in highway cruise. Auto engines will develop full rated power only at much higher rpm than aircraft engines, typically requiring gearing (additional complexity and weight) to turn the prop at subsonic speeds. These are some of the engineering challenges. Aircraft engines are actually well-designed for what they have to do: high power at relatively low rpm. What would make them more efficient is electronic timing, ignition, and mixture control. This is more or less where the various diesel engine design variants have gone.

Sure, automotive engines as they exist are not well suited for aircraft, but that doesn't mean a new gasoline engine that cannot be engineered with all these differences in mind. Maybe starting from automotive, maybe not. You can't tell me that 70+ year old engine design is the best we can do.

On the fuel side, aviation gasoline must meet different and more stringent standards for vapor pressure, etc. than automotive fuel for flight safety. In addition, Jet-A can be made in more abundance from crude petroleum than gasoline. Gasoline is a rather choice distillate from petroleum refinement. Jet-A is a more readily available fuel than gasoline many places.

That makes sense.

 

[RudyP]

I understand that automotive engines are not the same as aircraft. But if you are going to design(or adopt) a brand new engine(diesel to run on jet A), why not do it for gasoline? I cannot believe that it would be that difficult to adopt an existing unleaded automotive engine for aircraft use by a major manufacturer(Honda?).

If Honda set their mind to it, they could easily develop a GA engine running on autogas that would completely crush the TCM/Lycoming competition and sell nearly 1000 engines a year! ...And it wouldn't show up within the rounding error of their financials...

The issue isnt that what you are saying is not technically feasible, it is. The issue is that the tiny little GA market is utterly unattractive to any big boy engine makers.

Just looked it up and Honda sells about 5M cars every year as well as another 26+M other vehicles. So that is over 30M engines made per year. Or a little over 3500 engines per hour 24/7/365...

 

[Kenny Phillips]

I get that MoGas has been used on certain current engines. I also get that you can't use ethanol gas in current aircraft engines. But this is not what i'm talking about. I'm specifically referring to designing new engines for different fuel. It shouldn't be that difficult to get same level of power from 93 octane with a more modern engine design. And probably more reliability to boot. Ethanol or not.

Given that gasoline can be used for other things, it may be a lot less difficult to convince airport operators to install a tank for it.

Unless the FAA grants a blanket waiver on engine replacements, there's not enough money to bother with it. The number of new, larger-than-LSA piston aircraft that don't say "Cirrus" on the side is quite tiny.

 

[Ken Ibold]

I cannot believe that it would be that difficult to adopt an existing unleaded automotive engine for aircraft use by a major manufacturer(Honda?).

Porsche tried it = fail
Toyota tried it = fail

 

[genna]

Unless the FAA grants a blanket waiver on engine replacements, there's not enough money to bother with it. The number of new, larger-than-LSA piston aircraft that don't say "Cirrus" on the side is quite tiny.

Yet they do bother with trying to make diesel.

 

[bobmrg]

I get that MoGas has been used on certain current engines. I also get that you can't use ethanol gas in current aircraft engines. But this is not what i'm talking about. I'm specifically referring to designing new engines for different fuel. It shouldn't be that difficult to get same level of power from 93 octane with a more modern engine design. And probably more reliability to boot. Ethanol or not.

Given that gasoline can be used for other things, it may be a lot less difficult to convince airport operators to install a tank for it.

Developmental and certification costs would not be covered by sales to a relatively small market. "If it ain't broke, don't fix it."

Bob

 

[Ted]

At a basic level, the anti-knock characteristics of regular unleaded won't work in some of the higher horsepower engines (basically anything turbocharged, and 300+ HP naturally aspirated engines). This is a vast oversimplification of matters but it's close enough.

Most airports/FBOs don't want to stock more than one AvGas (be it 100LL, MoGas, whatever). So whatever they pick has to work for everything. Since the vast majority of the fleet is legacy and always will be, that change is a hard sell. The politics get very complex.

I've been involved in this for over 10 years now at this point. Honestly, 100LL isn't a huge problem. Diesels have their own problems, as discussed on another recent thread. The FAA is working on a drop-in replacement. It'll happen eventually.

Adapting automotive gasoline engines to aviation use and maintaining the performance and economy metrics is a lot harder than it sounds.

Keep on buying blue juice in the meantime. Or, if your engine/aircraft can support MoGas, go for that if you don't mind bringing it in jerry cans.

 

[bnt83]

Wanna run mogas? seems simple, install lower compression pistons and tweak the fuel metering system. Who knew it was so easy to solve that?

Oooopsie, I think that just invalidated all the performance data in the flight manuals, but who looks at that stuff anyways right?

Ability to burn mogas is a high selling point for some.

(There may be fuel systems that can't do it, and not even an engine issue at all)

 

[chemgeek]

You can't tell me that 70+ year old engine design is the best we can do.

You can look at this a different way: 70 years of engineering evolution has pushed aviation gasoline engines to a mature, robust design that suits their application, which requires relatively high power at relatively low output rpm. Porsche tried to adapt their 911 automotive engine to aviation, and it was not a resounding success, and achieved little market penetration. It is no longer supported. Rotax engines (which have automotive roots) on the other hand have had remarkable success in the lower power LSA market.

 

[bnt83]

I'm not in-lust with new engines (or imaginary ones) as the old "on condition" ones we have now will stay far cheaper than new type certified ones which manufacturers will keep a tight grip on and likely carry mandetory hourly and calender life limits.

 

[genna]

You can look at this a different way: 70 years of engineering evolution has pushed aviation gasoline engines to a mature, robust design that suits their application, which requires relatively high power at relatively low output rpm. Porsche tried to adapt their 911 automotive engine to aviation, and it was not a resounding success, and achieved little market penetration. It is no longer supported. Rotax engines (which have automotive roots) on the other hand have had remarkable success in the lower power LSA market.

My limited understanding with Porsche and Toyota(Lexus) AC engine attempts is that they were timed pretty badly. Porsche - a small company with its own financial problems at the time - made a pretty good engine that went into a few aircraft and at least one went into production. Then the industry tanked...

 

[Kenny Phillips]

Wanna run mogas? seems simple, install lower compression pistons and tweak the fuel metering system. Who knew it was so easy to solve that?

Oooopsie, I think that just invalidated all the performance data in the flight manuals, but who looks at that stuff anyways right?

Ability to burn mogas is a high selling point for some.

(There may be fuel systems that can't do it, and not even an engine issue at all)

And, of course, replace your valves more often.

 

[Bill]

Actually, the engines we use are pretty well optimized for aircraft use. Most of these engines were introduced in the early to mid-50's, after all of the development that was made into pistons engines for the war effort. They have fairly high Hp/weight and Hp/fuel consumption ratios.

 

[labbadabba]

Porsche tried it = fail
Toyota tried it = fail

Mercedes tried it = SUCCESS. Granted, it was a diesel which sorta blows up the OP's point...

 

[MauleSkinner]

My limited understanding with Porsche and Toyota(Lexus) AC engine attempts is that they were timed pretty badly. Porsche - a small company with its own financial problems at the time - made a pretty good engine that went into a few aircraft and at least one went into production. Then the industry tanked...

The industry is still tanked...nobody wants to go there.

 

[Ted]

Mercedes tried it = SUCCESS. Granted, it was a diesel which sorta blows up the OP's point...

Saying Mercedes tried it isn't exactly true, unless there's a development I'm not thinking of.

Thielert tried it using a Mercedes engine as a baseline with significant changes. Calling that a success would at best require squinting. The 1.7 had a lot of issues. When they went to the bigger engines it was a bit better, but ultimately pitifully low TBRs. The Austro seems to have done better than Thielert did, but still not without its issues. And major engine issues at that. Now Continental has the Thielert/Centurion lineup, and supposedly fixed some of the bugs, but as I said in the other thread you're still not seeing these replacing legacy piston engines in droves, or even in noticeable numbers.

Either way, Mercedes themselves didn't do those engines, they just sold the parts to other companies to build those engines.

 

[nrpetersen]

And, of course, replace your valves more often.

Auto and boat engines have solved the valve problems. I can remember traveling with a British engine consultant in the early 70s who predicted massive US auto engine problems with unleaded fuels to be introduced into the US market in 1975. It never happened. It does require hardened seats but that's not any problem. One of my MechEngr profs predicted we'd encounter exceptionally clean running engines with unleaded fuels.

Most existing Lycosaurus engines have an 80 octane version with lower compression pistons that could (and are) burning 91 Octane MoGas. The only operating "problems" are the lack of ignition and lead fouling difficulties.

The problem with E0 MoGas as universal for aircraft is the large number of high compression engines that the owners are trying to grasp any reason to justify ignoring an E0 MoGas solution.

 

[Let'sgoflying!]

Avgas is problematic. It's bad for environment

I probably just need dragging out from my cave but has this actually been substantiated or is it presumed? I mean has someone been able to show damage to wildlife populations or people etc?

 

[GRG55]

Yet they do bother with trying to make diesel.

Honda? Nope
Porche? Nope
Toyota? Nope

Not even Daimler, whose diesel automotive engine blocks and accessories seem to be a favourite starting point for some of the developers (Thielert, now Continental? and Austro).

It's inconceivable to me that the development effort that's been invested in aircraft diesel to date can ever be recovered from the minuscule numbers of engines being sold. At least Diamond has the advantage of selling two engines for every 42 or 62 that leaves the lot.

 

[Skibum44]

I probably just need dragging out from my cave but has this actually been substantiated or is it presumed? I mean has someone been able to show damage to wildlife populations or people etc?

It can’t even be a fraction of the unleaded impact on the environment. How about the lead acid batteries in every rig? I throw away at least 15 every year through my fleet. I like to burn AVgas... I guess I am extra bad for the environment. That said if I could get a affordable diesel in my x-country bird at a practical price I would jump on it at TBO.

 

[GRG55]

...The problem with E0 MoGas as universal for aircraft is the large number of high compression engines that the owners are trying to grasp any reason to justify ignoring an E0 MoGas solution.

LOL. Really?
I own two airplanes with a collective three engines that require 100LL. Maybe you could point me to the high compression piston owners organization or lobby group that is grasping for reasons to impede the use of mogas in these engines?

BTW, at any given time there are 45 or more different mogas blends in the system and being distributed/retailed in the Lower 48 (that number proliferates in summer). Part of the adoption problem is the inability to ensure a tight, consistent specification for mogas used in aircraft. And it's far more than ethanol oxygenates that differentiate them. So your so-called E0 in one place can't be assured to be the same as somewhere else in the Union.

 

[genna]

Honda? Nope
Porche? Nope
Toyota? Nope

Not even Daimler, whose diesel automotive engine blocks and accessories seem to be a favourite starting point for some of the developers (Thielert, now Continental? and Austro).

It's inconceivable to me that the development effort that's been invested in aircraft diesel to date can ever be recovered from the minuscule numbers of engines being sold. At least Diamond has the advantage of selling two engines for every 42 or 62 that leaves the lot.

I only mentioned Honda because they could. Clearly, someone(continental, austro) thinks there is a business case for non-avgas engines. So yes, someone bothers.

A valid point has been made about availability of gasoline at airports and certain gasoline properties that make its use problematic for aircraft. I suspect if that weren’t the case(along with some certification requirements relaxation), it would be pretty easy and cost effective for an auto manufacturer to jump in and create an engine.

Technology has progressed quite a lot since Porsche tried. Horsepower is easy, engines are very reliable and easily tested. And they often spend a lot of money on very limited production engines for halo cars. Not to mention that with prices for singles approaching 1mm, there would be room for recouping cost in new planes by selling a well supported and reliable engine

 

[flyer770]

It can’t even be a fraction of the unleaded impact on the environment. How about the lead acid batteries in every rig? I throw away at least 15 every year through my fleet. I like to burn AVgas... I guess I am extra bad for the environment. That said if I could get a affordable diesel in my x-country bird at a practical price I would jump on it at TBO.

I hope you’re turning in your batteries for the core charge. If you are, the batteries are being recycled.

 

[ktup-flyer]

Honda? Nope
Porche? Nope
Toyota? Nope

Not even Daimler, whose diesel automotive engine blocks and accessories seem to be a favourite starting point for some of the developers (Thielert, now Continental? and Austro).

It's inconceivable to me that the development effort that's been invested in aircraft diesel to date can ever be recovered from the minuscule numbers of engines being sold. At least Diamond has the advantage of selling two engines for every 42 or 62 that leaves the lot.

Porsche did have diesels in the Macan and Cayenne. Got axed after the MY2018

 

[GRG55]

I only mentioned Honda because they could. Clearly, someone(continental, austro) thinks there is a business case for non-avgas engines. So yes, someone bothers.

A valid point has been made about availability of gasoline at airports and certain gasoline properties that make its use problematic for aircraft. I suspect if that weren’t the case(along with some certification requirements relaxation), it would be pretty easy and cost effective for an auto manufacturer to jump in and create an engine.

Technology has progressed quite a lot since Porsche tried. Horsepower is easy, engines are very reliable and easily tested. And they often spend a lot of money on very limited production engines for halo cars. Not to mention that with prices for singles approaching 1mm, there would be room for recouping cost in new planes by selling a well supported and reliable engine

Anybody here know which, if any, new production aircraft Continental is supplying Diesel engines for?

It hasn't gone unnoticed that the largest manufacturer of single piston aircraft, including those delightful models approaching $1 million, has to date never offered one with a diesel or mogas fuelled motor.

If it truly was as "easy" as you think, it would have already been done. It isn't, and that's why it hasn't.

 

[RudyP]

I only mentioned Honda because they could. Clearly, someone(continental, austro) thinks there is a business case for non-avgas engines. So yes, someone bothers.

Someone cares - someone who is a small-time player with aviation in their blood and dreams in their eyes (e.g., Thielert, NOT Daimler Benz). Those people are willing to try but they don't have the same resources as the real engine companies you are referring to and they usually fail.

Big, established companies like Honda do NOT care one whit about GA piston engines. Why on earth would Honda go to the trouble of picking up a whole new line of liability exposure for a TOTAL market that represents ANNUALLY the equivalent of about 10 minutes of production line run time for them? They could completely put TCI and Lycoming out of business and capture 100% of the GA piston market and it still would only be <0.01% of their 2018 revenue.

 

[Ted]

I own two airplanes with a collective three engines that require 100LL. Maybe you could point me to the high compression piston owners organization or lobby group that is grasping for reasons to impede the use of mogas in these engines?

Your Aztec engines could run on MoGas just fine in my professional opinion. In fact those engines are approved for 91/96, which is pretty close to 93 AKI. I wouldn't worry about 91 either. But that's from a technical perspective, and focused solely on the engine. It does not address a legal perspective nor does it address the airframe side, which is often where you have more issues when using MoGas.

Ultimately the only ones who think it's "easy" to do the auto conversion are people who've had zero involvement in actually doing it.

 

[Dan Thomas]

Ultimately the only ones who think it's "easy" to do the auto conversion are people who've had zero involvement in actually doing it.

That there. I've had experience with a Subaru conversion, and spent at least 40 hours adapting it and tinkering with it for every hour in flight. If I had to get certificaton? You'd never, ever make any money on it.

 

[Kenny Phillips]

Your Aztec engines could run on MoGas just fine in my professional opinion. In fact those engines are approved for 91/96, which is pretty close to 93 AKI. I wouldn't worry about 91 either. But that's from a technical perspective, and focused solely on the engine. It does not address a legal perspective nor does it address the airframe side, which is often where you have more issues when using MoGas.

Ultimately the only ones who think it's "easy" to do the auto conversion are people who've had zero involvement in actually doing it.

This was discussed at a safety seminar I attended a couple of decades ago; the only thing that has really changed is the addition of ethanol (which is not allowed in what we call "mogas".)
The engines on darn near every plane would run on mogas, but mogas has a higher vapor pressure in general (and it's not regulated), and has no lead. In the short term, at low altitudes, it may get by, depending on the fuel system.

 

[Kenny Phillips]

That there. I've had experience with a Subaru conversion, and spent at least 40 hours adapting it and tinkering with it for every hour in flight. If I had to get certificaton? You'd never, ever make any money on it.

I'm thinking of building a Zenith with a Subie conversion. I know from the bad experience of others that torsional vibration is waiting to hammer your gearbox to bits, unless accounted for and suitably quelled in the design. For me, it's just because it's a Subaru; plan B is a certified engine, of course!

 

[Ted]

This was discussed at a safety seminar I attended a couple of decades ago; the only thing that has really changed is the addition of ethanol (which is not allowed in what we call "mogas".)
The engines on darn near every plane would run on mogas, but mogas has a higher vapor pressure in general (and it's not regulated), and has no lead. In the short term, at low altitudes, it may get by, depending on the fuel system.

I happen to know a few things about this subject matter seeing as it comprised much of my duties in my job for several years.

Starting? Sure, any of these engines will start on MoGas.

Running at power? Good luck on a Navajo. You'll detonate so much you'll melt the cylinders. Same for a high compression angle valve 540, a 520/550, and especially TSIO/GTSIO-520s/550s. Keep in mind this makes up a lot of the fleet that can't run on MoGas. Yeah, you could add water injection or the like to help with high power operations (and there's a company that has STC'd water injection for some aircraft to allow use of MoGas) but that's a lot of effort and expense. Very poor payback.

The vapor pressure is the big issue. RV guys run into this all the time, where running on MoGas in some aircraft will have issues. Of my friends who run MoGas in RVs most of them have reported some issue at some point and one tank on 100LL and one tank on MoGas is the more common combination. That works for an experimental but you really can't certify that.

If I were building an aircraft from the ground up, I know how I'd design the fuel system to eliminate these issues. It'd look a lot like an automotive fuel system from the 90s. But the reality is that it doesn't make a lot of sense.

 

[Dan Thomas]

I'm thinking of building a Zenith with a Subie conversion. I know from the bad experience of others that torsional vibration is waiting to hammer your gearbox to bits, unless accounted for and suitably quelled in the design. For me, it's just because it's a Subaru; plan B is a certified engine, of course!

The certified engine would have cost less in the end, believe me. And the performance would have been better. The Subaru has a 5600 RPM redline, but with the reduction the prop runs at around 2550 at that speed. It did not like cruising anywhere near that redline; the noise and fuel consumption was awesome, even leaned out. If you lean it aggressively it will burn valves instantly. Subaru's valves are tiny and depend on EFI to keep them happy. So anyway, the cruise of this Glastar with the 130 HP Soob was only 100 MPH or so at 4600 RPM. The 135-HP Lyc it was designed for could be cruised at anything up to redline (2700 RPM), and at 75% power it cruised at 130 MPH.

Converting an engine? It's not nearly as simple or as good as it sounds. Some guys have done it successfully, but it takes a lot of time and money.

 

[genna]

Someone cares - someone who is a small-time player with aviation in their blood and dreams in their eyes (e.g., Thielert, NOT Daimler Benz). Those people are willing to try but they don't have the same resources as the real engine companies you are referring to and they usually fail.

Big, established companies like Honda do NOT care one whit about GA piston engines. Why on earth would Honda go to the trouble of picking up a whole new line of liability exposure for a TOTAL market that represents ANNUALLY the equivalent of about 10 minutes of production line run time for them? They could completely put TCI and Lycoming out of business and capture 100% of the GA piston market and it still would only be <0.01% of their 2018 revenue.

That's interesting, but .... Honda made a jet, yet, at least in USA, new GA piston engine planes outsell GA jets about 2-1. So far they produced about 90 of them... I took them nearly 20 years to make the damn thing

Sure, they make more $$ on each jet than they may on a piston engine. But that emboldened sentence may not be completely accurate.