Questions about AC piston engines that I've never dared to ask
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Questions about AC piston engines that I've never dared to ask
I would be thankful for any knowledgeable responses to this, apologies if already posted and answered, have searched:
(Just a note, when talking car engines, I'm talking about pre-1990's engines with a carburettor. Similarly, when talking AC engines, I'm talking about the Briggs and Stratton league engines that are powering the prehistoric aircraft that I fly :-)
1. How come that a typical 320 cubic inch (5244 cubic cm) SEPL aircraft engine only produces between 140 - 160 horsepower ? A typical European car with half the displacement produces the same power ! Is it the American tradition of "big, heavy, low power" engines" ? (Ever rented a a 6 cyl 3.0 L American car and wondered what was going on out there (if anything but noise) ?). Or, is it that it needs to operate at a relatively low RPM, like a ship's engine ? Please don't gimme the "reliability story", or the "AC engines operate at max power most of the time story", even a VW Polo 1.0 can "cruise" at 150 km/h for 250 000 km @ 7000 rpm, just see how the Germans drive them on the autobahn !
2. How come that these engines are so vulnerable to carburettor icing ? Cars have been driving around at varous altitudes for 100 years without this problem, or ?
3. How come that car engines haven't had a leaning mechanism ? The same engines are running in Amsterdam (height: 0ft) and Madrid (height: many ft :-)
4. I often hear that "torque" is what's important in AC engines, not horse-power. Just wonderig what the difference is between a VW Polo 1.0 plowing the autobahn at 150 km/h and a Piper Cherokee plowing through the air at 100 Kts ?
BTW, I had an interesting experience some months ago. My friend and I started talking engines, I bragged about the "huge" engines in the SEP aircraft I fly. He has a Porche 944 S2 (3.0L, 211 HP, reportedly the largest 4 cyl car engine ever made). Ended up with us test driving / flying each other's vehicles. I had the most fun !
(Just a note, when talking car engines, I'm talking about pre-1990's engines with a carburettor. Similarly, when talking AC engines, I'm talking about the Briggs and Stratton league engines that are powering the prehistoric aircraft that I fly :-)
1. How come that a typical 320 cubic inch (5244 cubic cm) SEPL aircraft engine only produces between 140 - 160 horsepower ? A typical European car with half the displacement produces the same power ! Is it the American tradition of "big, heavy, low power" engines" ? (Ever rented a a 6 cyl 3.0 L American car and wondered what was going on out there (if anything but noise) ?). Or, is it that it needs to operate at a relatively low RPM, like a ship's engine ? Please don't gimme the "reliability story", or the "AC engines operate at max power most of the time story", even a VW Polo 1.0 can "cruise" at 150 km/h for 250 000 km @ 7000 rpm, just see how the Germans drive them on the autobahn !
2. How come that these engines are so vulnerable to carburettor icing ? Cars have been driving around at varous altitudes for 100 years without this problem, or ?
3. How come that car engines haven't had a leaning mechanism ? The same engines are running in Amsterdam (height: 0ft) and Madrid (height: many ft :-)
4. I often hear that "torque" is what's important in AC engines, not horse-power. Just wonderig what the difference is between a VW Polo 1.0 plowing the autobahn at 150 km/h and a Piper Cherokee plowing through the air at 100 Kts ?
BTW, I had an interesting experience some months ago. My friend and I started talking engines, I bragged about the "huge" engines in the SEP aircraft I fly. He has a Porche 944 S2 (3.0L, 211 HP, reportedly the largest 4 cyl car engine ever made). Ended up with us test driving / flying each other's vehicles. I had the most fun !
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GA engines with carburetors normally are of the updraft variety, and as a result can sometimes experience carburetor icing.
Altho of the downdraft variety, many older cars with carbs also experienced icing...the Buick straight and the Chevrolet straight six, are two such examples.
Piston engines with pressure carburetors almost never experience icing...all large radials (CurtisWright turbocompound engines excepted, fuel injection) were thus equipped.
I've personally flown many...no carb icing problems that I can recall.
Regarding leaning at altitude with cars equipped with carburetors, years ago such cars sold in Denver (for example, elevation 5280 feet) were especially equipped with smaller jets, and were especially designed for such heights.
As a result, they ran quite lean lower down...poor results were obtained, unless you changed the jets.
Altho of the downdraft variety, many older cars with carbs also experienced icing...the Buick straight and the Chevrolet straight six, are two such examples.
Piston engines with pressure carburetors almost never experience icing...all large radials (CurtisWright turbocompound engines excepted, fuel injection) were thus equipped.
I've personally flown many...no carb icing problems that I can recall.
Regarding leaning at altitude with cars equipped with carburetors, years ago such cars sold in Denver (for example, elevation 5280 feet) were especially equipped with smaller jets, and were especially designed for such heights.
As a result, they ran quite lean lower down...poor results were obtained, unless you changed the jets.
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Aircraft Piston Engines v's Auto engines
Hi Gargleblaster.Your questions, in a sense, are asking for comparisons between apples and oranges. In regard to the question of the large capacity aero engines in the aircraft you fly, they are generally of the low compression variety designed to operate with lower octane fuels. They are, by comparison to the auto engines mentioned, low revving with maximum speed usually not higher than 2750-2850 RPM. By the way, piston engines in aircraft do not operate at max power for most of the time, nor do they operate at max RPM for long periods. During Take-off and climb are the periods when high RPM/high power is used most.
One of the reasons aircraft engines are susceptible to icing is the environment in which they operate. There are several types of icing which can have an impact on engine performance. Impact icing occurs when super cooled droplets impact with the air intake and usually this type of icing is not removable with heat. Throttle icing and/or fuel icing can occur without any visible moisture present. As the air or fuel/air misture passes through the carburettor the pressure drops and so does the temperature within the carburettor. Throttle and fuel icing will occur with temps from say -10 to +20, especially with high humidity and low power settings. (This is one of the reasons it is prudent to select Carby Heat Full and increase RPM regularly during low power descents) Use of full carby heat will usually clear this type of icing. One good rule is "If you need carburettor heat, give it the full heat"
Your question regarding "torque" v's Horsepower is interesting. The important thing is to have RPM available. With a fixed pitch propeller there is no provision to vary RPM other than with the throttle or by changing airspeed. The greater the RPM the greater the thrust, or if you like, the mass air flow. Horsepower and torque, as it applies to aircraft, are very much inter-related.
I am unaware of any carburettor equipped cars with mixture controls. As you probably know aeroplanes can have manual or automatic mixture controls, however the previous post by 411A says it all. Re-jetting is the conventional way with cars, although most would just accept the richer mixture at higher elevations.
As for your question regarding what is different between a VW travelling at 150 KPH v's a Piper flying at 100 MPH I suppose the flipant answer would have to be "about 10 KPH and varying degrees of altitude".
Hope this ramble helps.
Cheers
Old Fella.
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Re 2)
I had a part share in a Hillman Imp 'boys banger' years back that ALWAYS used to suffer carb icing as it hit the wet air of the south west Wales coast line on its way to RAF Brawdy. By the time the 9 of us had stumbled out and lifted the lid there was nothing to show and it started perfectly.
I had a part share in a Hillman Imp 'boys banger' years back that ALWAYS used to suffer carb icing as it hit the wet air of the south west Wales coast line on its way to RAF Brawdy. By the time the 9 of us had stumbled out
and lifted the lid there was nothing to show and it started perfectly.
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When still a lad I remember my father had an 1930 something Morris and after that a 1930 something Alvis, both had a number of controls at the centre of the steering wheel, including mixture, advance and retard and can't remember the other one.
Only nine in an Imp BOAC? You must have all been rather large!
Only nine in an Imp BOAC? You must have all been rather large!
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A/C piston v Auto piston
Hi Gargleblaster
As I understand the reason for the big difference in cubic capacity in auto and aeros is that the prop' rpm is restricted because if it was to Max rpm the same as an auto engine at @ 6000 rpm the tips of the prop blades would be supersonic which would not work. So the prop pitch is designed to operate in a rpm band half that of an auto engine approx. so we need to be able to produce the power required at approx half auto rpm so hence the much bigger cubic capacity.
Your question on carb ice is a good one. I used to drive an old volvo 340 (uk) with a renault engine and it used to lose power on a long run if I was giving it wigins (relative term for a 340).One day I eventually saw the evidence. I had been driving the car with the air cleaner removed although i can,t remember why. when the car stopped i got out and looked under the bonnet into the carb and i could see this thing like a large polo mint. the car was not running and very shorty the polo mint came off the carb wall and shorly fell apart and dissappeared.Easier to believe when you have seen it!
As I understand the reason for the big difference in cubic capacity in auto and aeros is that the prop' rpm is restricted because if it was to Max rpm the same as an auto engine at @ 6000 rpm the tips of the prop blades would be supersonic which would not work. So the prop pitch is designed to operate in a rpm band half that of an auto engine approx. so we need to be able to produce the power required at approx half auto rpm so hence the much bigger cubic capacity.
Your question on carb ice is a good one. I used to drive an old volvo 340 (uk) with a renault engine and it used to lose power on a long run if I was giving it wigins (relative term for a 340).One day I eventually saw the evidence. I had been driving the car with the air cleaner removed although i can,t remember why. when the car stopped i got out and looked under the bonnet into the carb and i could see this thing like a large polo mint. the car was not running and very shorty the polo mint came off the carb wall and shorly fell apart and dissappeared.Easier to believe when you have seen it!
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Piston engines, just before the move to turbines, were producing around 60 hp per litre compared to 70 for my Vauxhaul Omega Ecotec engine. The things you see on single-engined aeroplanes are from an american era where it was a lot simpler to have huge capacity. Fuel was cheap and a low stressed engine has all sorts of benefits.
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Originally Posted by parabellum
Only nine in an Imp BOAC? You must have all been rather large!
Thread Starter
Thanks for all the good information !
May I ask a bonus question ?
I was taught to always drain immediately after fueling. Shouldn't I instead drain as the very first thing before the aircraft is moved at all because movement and pouring fuel into the tanks must stir-up any water in the tanks ?
May I ask a bonus question ?
I was taught to always drain immediately after fueling. Shouldn't I instead drain as the very first thing before the aircraft is moved at all because movement and pouring fuel into the tanks must stir-up any water in the tanks ?
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I was taught to always drain immediately after fueling. Shouldn't I instead drain as the very first thing before the aircraft is moved at all because movement and pouring fuel into the tanks must stir-up any water in the tanks ?
With turbine aircraft this is not especially necessary, however with these aircraft, the 'penny-in-the pail' technique, prior to refueling is many times practiced by older biz-jet crews.
Also highly recommended.
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My TR7 ran beautifully at sea level in Dover and at the top of the Stelvio pass in Italy (10,000 feet or so) - all done automatically by a pair of SU carbs with a basic design that is nearly 100 years old. That car had a Dolomite Sprint engine and 140bhp from 2 litres - and despite being a modified engine, the standard carbs could adjust automatically.
Perhaps a better question would be:
"Why in this day and age don't aero engines have automatic mixture control?"
Re: Icing:
Many carb equipped cars have water or electrically heated inlet manifolds - and even if they don't, the engine bay is usually warm enough to prevent icing thanks to heat from the radiator and/or exhaust manifold.
Re: Power:
Power (for a given cylinder/head/inlet/exhaust design) is, to all intents and purposes a factor of capacity and RPM - so the low RPM of a big aero engine means that they are never going to produce the same power as a faster revving car engine of the same size. BHP/litre on a VW flat four leaves Lycoming in the shade - thanks to the extra RPM for the most part.
Last edited by moggiee; 10th Mar 2008 at 02:06.
My answers:
1. Power is rated by bore x srtoke x compression ratio x RPM. Comparing the two, aero engines are way down on RPM by necessity of the prop RPM and they tend to have much lower compression ratios. Compare a Lycoming to one of the more modern Rotax engines. The Rotax has gearing and car type compression ratios and acheives power for much less volume. Also consider the types of cars which were around when these engines were designed. (I know your mate's Porche is a bad example because it's engine is basicly the same design as an aero engine). They produced far less power then. Aero engine developement is hindered by certification which doesn't affect automotive engines.
2. Car engines with carburetters are vulnerable to carb icing. (Actually, hardly any cars have carbs now as fuel injection is the norm as a result of emission controls). Some designs are less than others, and some cars have a hot water chamber in the carb. I had a FIAT Uno which suffered badly, particularly on misty days in the UK when the temp was about 10 degrees.. It had a 'winter /summer' switch on the air inlet which drew warmer air from close to the exaust manifold - a basic and ineffective fix. The guy I bought it from had the engine elctronic management system replaced twice at great expense trying to fix the problem. He looked a bit sheepish when I told him what the problem really was, particularly as he was an RAF Central Flying School examiner and should have known all about carb icing!
3. The range of pressures an aro engine is exposed to is usually much greater than the auto engine, so a lot of manufactureres thought it not necessary to add mixture contol. However, this is not always the case. As mentioned, the SUs and Stromberg carbs did have a rudimentary mixture control, but to be more acuare it was a pressure balance system. This is why MGs with their SUs had a good reputation in the Alps. Some more conventional carbs did have mixture contol. Some of the high end Webers had an aneroid capsule which altered the jet. Most modern injection systems have a pressure transducer to adjust fule flow for ambient conditions. They need it to pass the emissions legislation.
4. Torque is a measurement of 'twisting power' to put it simply. Large aero engines have masses of it - they would have to to turn the prop. Your small auto engine doesn't need so much as it rotates a narrow shaft input to a gear box. However, some auto advertising agencies like to tell you how much torque a car has. It's largely irrelevant unless it's a sports car with lots of acceleration. But torque in a car can be great fun. My TR6 had lots of it!
1. Power is rated by bore x srtoke x compression ratio x RPM. Comparing the two, aero engines are way down on RPM by necessity of the prop RPM and they tend to have much lower compression ratios. Compare a Lycoming to one of the more modern Rotax engines. The Rotax has gearing and car type compression ratios and acheives power for much less volume. Also consider the types of cars which were around when these engines were designed. (I know your mate's Porche is a bad example because it's engine is basicly the same design as an aero engine). They produced far less power then. Aero engine developement is hindered by certification which doesn't affect automotive engines.
2. Car engines with carburetters are vulnerable to carb icing. (Actually, hardly any cars have carbs now as fuel injection is the norm as a result of emission controls). Some designs are less than others, and some cars have a hot water chamber in the carb. I had a FIAT Uno which suffered badly, particularly on misty days in the UK when the temp was about 10 degrees.. It had a 'winter /summer' switch on the air inlet which drew warmer air from close to the exaust manifold - a basic and ineffective fix. The guy I bought it from had the engine elctronic management system replaced twice at great expense trying to fix the problem. He looked a bit sheepish when I told him what the problem really was, particularly as he was an RAF Central Flying School examiner and should have known all about carb icing!
3. The range of pressures an aro engine is exposed to is usually much greater than the auto engine, so a lot of manufactureres thought it not necessary to add mixture contol. However, this is not always the case. As mentioned, the SUs and Stromberg carbs did have a rudimentary mixture control, but to be more acuare it was a pressure balance system. This is why MGs with their SUs had a good reputation in the Alps. Some more conventional carbs did have mixture contol. Some of the high end Webers had an aneroid capsule which altered the jet. Most modern injection systems have a pressure transducer to adjust fule flow for ambient conditions. They need it to pass the emissions legislation.
4. Torque is a measurement of 'twisting power' to put it simply. Large aero engines have masses of it - they would have to to turn the prop. Your small auto engine doesn't need so much as it rotates a narrow shaft input to a gear box. However, some auto advertising agencies like to tell you how much torque a car has. It's largely irrelevant unless it's a sports car with lots of acceleration. But torque in a car can be great fun. My TR6 had lots of it!
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penny-in-the pail
Not heard of it 411A. Care to elaborate please. (Flown turbines all my life)
Not heard of it 411A. Care to elaborate please. (Flown turbines all my life)
Place the penny at the bottom of the pail then fill the pail with jet fuel.
If you can read the date on the penny through the fuel...good to go.
Very effective, and was used by many JetStar, Sabreliner and Lear pilots years ago....and still sometimes noticed in use today.
Carb Icing
One point that I haven't seen mentioned here (unless I missed it in skimming) is to do with the balance between carb heat and mixture.
Aero piston engines are intended to run rich to keep cylinder head temperatures down (reliability deemed more important than economy for aeroplanes). Applying carb heat makes the mixture richer still because hot air is less dense than cold. The risk at high power setting is therefore to overrich the mixture causing cut-out, and so should be avoided.Hence the carb heat tends to be placed at the manual discretion of the pilot whilst in cars it is either set as a seasonal adjustment under the bonnet, or more likely these days it is thermostatic.</p>
Aero piston engines are intended to run rich to keep cylinder head temperatures down (reliability deemed more important than economy for aeroplanes). Applying carb heat makes the mixture richer still because hot air is less dense than cold. The risk at high power setting is therefore to overrich the mixture causing cut-out, and so should be avoided.Hence the carb heat tends to be placed at the manual discretion of the pilot whilst in cars it is either set as a seasonal adjustment under the bonnet, or more likely these days it is thermostatic.</p>
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Aero piston engines are intended to run rich to keep cylinder head temperatures down
http://www.avweb.com/news/pelican/182146-1.html
controls at the centre of the steering wheel, including mixture, advance and retard and can't remember the other one.
Large cars (especially American) in the 1950s also had the gearstick mounted on the steering column ("column change").
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Or (for the real mechanical buffs) the gear control quadrant for a preselector gearbox. 10 bonus points if you know how to drive one of those
Sorry, but the left pedal was not a clutch. If you used it as such you got into problems, which was a common preselector issue for those used to conventional transmissions.
The type of buses used in London until the 1970s (the RT type) had these fitted, and you could watch the differences the driver had to do through the window behind. There was a hiss every time the pedal was pressed as it was air-powered. At the bus stop, flick it into second (first being very low), stamp on the pedal, flick it into third - and all this before the bus started, for which you just pressed the accelerator.
Lets get back to aircraft. I have a question about piston engines. The Centaurus engine (Airspeed Ambassador and others) was a sleeve-valve engine. How did these work ? Was this behind the huge cloud of smoke on startup after standing overnight ?
The type of buses used in London until the 1970s (the RT type) had these fitted, and you could watch the differences the driver had to do through the window behind. There was a hiss every time the pedal was pressed as it was air-powered. At the bus stop, flick it into second (first being very low), stamp on the pedal, flick it into third - and all this before the bus started, for which you just pressed the accelerator.
Lets get back to aircraft. I have a question about piston engines. The Centaurus engine (Airspeed Ambassador and others) was a sleeve-valve engine. How did these work ? Was this behind the huge cloud of smoke on startup after standing overnight ?