Fuel Press Question

[twright]

Hi there

Really enjoying the Cherokee. I fly PA28s in real life so this was a welcome addition to my hangar!

Just a quick query when starting the engine - when you switch the fuel pump on, the pressure builds and then you switch it off for starting. In the real aircraft, the fuel pressure stays constant, however in the A2A Cherokee it falls as soon as the pump is switched off. Is this intentional?

Kind regards
Tom Wright

[Great Ozzie]

Just a quick query when starting the engine - when you switch the fuel pump on, the pressure builds and then you switch it off for starting. In the real aircraft, the fuel pressure stays constant, however in the A2A Cherokee it falls as soon as the pump is switched off. Is this intentional?

Hi Tom,

I normally left the fuel pump on (off during the ground run-up check) - but this comes from Warriors... (i.e. just a fwiw).

Back on topic... I remember the fuel pressure "staying pressurized" if the pump was turned off with the engine off, say during a pre-start check. I think this was brought up some time ago and I think someone was going to check... maybe that hasn't happened yet or made its way into the upgrade patch.

-Rob

[Scott - A2A]

...in the A2A Cherokee it falls as soon as the pump is switched off. Is this intentional?

Yes, this is intentional. The fuel pressure drops off in the Cherokee's we tested shortly after the fuel pump is turned off. It's the same with our Comanche.

Scott.

[AKar]

This would depend on a few factors, namely those that contribute any leaking from the fuel line to somewhere, mainly into the intake through the carburetor, and on where the plunger in the pump happens to stop - it is the plunger's spring pressure againat the carburetor float valve that keeps up any fuel pressure in the system for any prolonged duration of time. I would figure whether you've got your mixture cut or in and so on can all make a bit of difference.

-Esa

[Nick - A2A]

I normally left the fuel pump on (off during the ground run-up check) - but this comes from Warriors... (i.e. just a fwiw).

This topic and the mention of different procedures for starting a PA-28 got me thinking... So, when did Piper get round to sticking fuel injected engines in the things?

A quick look on t'internet and the answer it seems is that they haven't got round to it yet! Except for the retractable gear Arrows with their IO-360s, brand new Cherokees rolling off the production line in 2015 still don't seem to have a fuel-injected engine sittin' under their hood (to misquote the Beach Boys lyrics! )

I guess the answer to why is summed up in one word: "certification". However, as a layman it seems odd that it's possible to buy a new aeroplane costing—I'd guess more than $300,000—which is fitted with a carburettor, when even the cheapest new cars available (in the West anyway) come with multiport fuel injection or similar.

Nick

[Great Ozzie]

Having a look at the type certificates... the Cherokee six used an IO-540 starting in '66 and in the 180hp Arrow in '67.

[Piper_EEWL]

However, as a layman it seems odd that it's possible to buy a new aeroplane costing—I'd guess more than $300,000—which is fitted with a carburettor, when even the cheapest new cars available (in the West anyway) come with multiport fuel injection or similar.

And why is that? What's the major advantage of a fuel injected aeroplane engine over the carbureted one? Let's say the the Lycoming O-360 compared to the IO-360?? They're both producing 180hp. They both have a TBO of 2000hrs. The carbureted one is even a bit lighter according to the Lycoming website (though it's only something around 5lbs). I don't think there's a major advantage in fuel consumption either. So I guess it's a philosophy question. Or isn't it?

[AKar]

Yes, none of the 'basic' Pipers have been commonly using fuel injection. Seneca uses TSIO-360 series from Continental and Arrow uses Lycoming's IO-360 series. But even Seneca is already a six-seater.

Just about the only advantages of carburetor that I can think of are related to simplicity and 'field-repairability', in everything else the fuel injection scores.

-Esa

[Piper_EEWL]

But how exactly do the fuel injected engines score? Not more power not less weight and not considerably less fuel consumption. So I wonder what's the advantage?

[AKar]

The biggest potential gains are due to better mixture distribution even a rudimentary fuel injection can provide. If it was better utilized, it would allow for modest fuel efficiency increases by enabling more leaning for cruise flight, and all the other benefits that even feeding of the cylinders provides. This is not really pursued by the manufacturers in naturally aspirated airplanes, however, it becomes more critical in case of turbocharged engines. One may indeed note that turbocharged, carbureted engines are very rare in GA airplane use.

-Esa

[Nick - A2A]

But how exactly do the fuel injected engines score?

Their other big advantage of course (and one that's demonstrated regularly here in the Accu-Sim versions) is that they're much less susceptible to induction icing.

When Piper first started putting the IO-360 in the Arrow, I gather it was really just because they couldn't find room for a carb and a retractable nosewheel so I suppose a more compact engine installation is another advantage of the fuel injected Lycos. Incidentally, when I was searching for the Beach Boys lyrics above (to make sure I misquoted them properly ) I came across this article from 1982 which addresses that question.

However, it's nearly 35 years since the article was written yet still the basic O-360 lives on!

Nick

[AKar]

But how exactly do the fuel injected engines score?

Their other big advantage of course (and one that's demonstrated regularly here in the Accu-Sim versions) is that they're much less susceptible to induction icing.

That's true also, albeit fuel injected Lycomings are somewhat prone to fuel injector icing, at close to freezing temperatures with high humidity. It is not too common, mainly because these planes tend to avoid those conditions anyway, but still quite possible. This is because the fuel injector servo uses venturi and throttle valve very much like carburetor does, it is not principally different in those respects.

Continental uses entirely different system, so they avoid this specific issue.

-Esa

[Piper_EEWL]

The biggest potential gains are due to better mixture distribution even a rudimentary fuel injection can provide. If it was better utilized, it would allow for modest fuel efficiency increases by enabling more leaning for cruise flight, and all the other benefits that even feeding of the cylinders provides. This is not really pursued by the manufacturers in naturally aspirated airplanes, however, it becomes more critical in case of turbocharged engines. One may indeed note that turbocharged, carbureted engines are very rare in GA airplane use.

Ok. I understand that fuel injection has a lot of advantages in car or motorcycle engines. Bu it doesn't seem like that the potentials fuel injection has are used in GA engines. I guess in turbocharged engines it's more critical since those engines are more prone to detonation? So it's more important to get a more accurate fuel air mixture per cylinder than it would be in a naturally aspirated engine?

Their other big advantage of course (and one that's demonstrated regularly here in the Accu-Sim versions) is that they're much less susceptible to induction icing.

Well yes that's true. But in RL VFR flying I don't find that to be a major drawback and there's tons of GA airplanes with carbureted engines out there that are operated in IFR (e.g. Scott's Comanche) and that still works

When Piper first started putting the IO-360 in the Arrow, I gather it was really just because they couldn't find room for a carb and a retractable nosewheel so I suppose a more compact engine installation is another advantage of the fuel injected Lycos.

Ok. I can see that be a real reason. Since the IO-360 seems to be a little bit less in height it would be the natural choice to go to if you have problems with the available space.

However, it's nearly 35 years since the article was written yet still the basic O-360 lives on!

That's exactly the point I was trying to make. The advantages of the fuel injected engines don't seem to outweigh the costs for the aircraft manufacturer (in this case Piper) to get the injected engines certified. Though the engines themselves are widely used (e.g. Cessna C172S) already and therefore are not only certified and have a good reputation but also the maintenance procedures are implemented world wide. Piper even has experience with the engine in the Arrow. So everything would be there for Piper but they're still using the O-360 in the Archer and the Seminole. So I guess it is a question of philosophy after all.

Great discussion guys.

[AKar]

I don't know what is the particular reason for Piper to use carbureted engines to the extend they use. I find it difficult to believe the certification would be the issue - these engines are all from the damned fifties anyway, and Piper is playing around with diesel stuff for instance.

Even if the fuel injected engines are very rudimentary, they do have their benefits, and it appears that solely in fuel consumption the benefits can easily be in range of 1 gph in good stock engines if leaned for economy. That's some US$400 (much more in many other places, like here in Europe) every 100 flight hours in savings in airplanes used much for trip flying, spending much time in cruise. Carbureted engines can be so poor in fuel distribution that they start to run shaky when specific fuel consumption would still drop far more with further leaning.

In turbocharged engines with poor fuel distribution, you'd just run your monitored cylinder at peak TIT (if using Lycoming's leaning method) while the others would group wherever they happened. And even if the roughness wasn't unbearable, then it would be likely that some of the richer cylinders are running unacceptably hot when cruising at high power - of course not noticed by a single-point CHT gauge - asking for some top problems. Of course the same happens with naturally aspirated engines, they just are generally less 'pushed' in cruise and cruise climb flight, operating at lower MPs.

-Esa

[Piper_EEWL]

That's true. And yes you're right you can purchase the Piper Archer with the Diesel engine as a factory option now can't you. And I have to say it makes much more sense than the fuel injected gasoline engine. The Diesel gives you such a good fuel economy and the fuel itself is dirt cheap compared to Avgas. A lot of manufacturers go down that path (Cessna was working on getting the Skylane certified with a Diesel though I don't know what the status on this is, Diamond Aircraft offers the DA40 with a Diesel as a factory option and so does New Robin Aircraft with the DR400). And with Avgas getting rarer since it's almost exclusively used for GA this seems to be a logical step. Though in Europe you can get conversions to regular unleaded fuel from Avgas. That already brings down your fuel costs by approximately 10-20% I don't know if they offer those kind of conversions in the States too?!

I guess the problem is that in Clubs and Charter Aircraft the pilots tend not to lean correctly or to max economy since usually the charter price is including fuel and pilots are afraid to lean to far and run the engine to lean. That's probably a reason why the fuel economy advantage of a fuel injected engine won't be as noticeable in those environments. Of course it's a different story for private operation.

In turbocharged engines with poor fuel distribution, you'd just run your monitored cylinder at peak TIT (if using Lycoming's leaning method) while the others would group wherever they happened. And even if the roughness wasn't unbearable, then it would be likely that some of the richer cylinders are running unacceptably hot when cruising at high power - of course not noticed by a single-point CHT gauge - asking for some top problems. Of course the same happens with naturally aspirated engines, they just are generally less 'pushed' in cruise and cruise climb flight, operating at lower MPs.

Got you

Edit:

I just read up on the Cessna Skylane JT-A Diesel engine project and it seems to be on hold since May 2015 due to some technical and certification issues. That's why the 182T with the gasoline Lycoming has returned into production according to this AOPA article issued in May this year:

http://www.aopa.org/News-and-Video/All- ... T-A-orders

If the article is correct Cessna is also working on a Diesel powered C172 which I'm sure will be a big hit with flight schools. There are already some retrofit versions available as this one for example:

http://www.tadistributors.com/convertin ... 2-skyhawk/

So maybe we can see an Accusim Diesel conversion one day

Anyways going way of topic here (again)