Engines: Carbureted vs Fuel Injected
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Engines: Carbureted vs Fuel Injected
I did use the search function, but amazingly, could find no thread talking about the pluses and minuses of carbureted engines as opposed to fuel injected ones. I saw on the hugely entertaining "Ferry Bad Experience" thread in bizjets the details of a 2007 PA28; as well as twin 430s, a glass cockpit, and aircon it had...a carbureted engine!
To me, this seems like archaic technology; not that fuel injection is exactly new tech, but I'm surprised to see a new aircraft not fitted with a fuel injected engine. Are there some benefits to running with a carburettor that make up for the hassle of carb icing?
To me, this seems like archaic technology; not that fuel injection is exactly new tech, but I'm surprised to see a new aircraft not fitted with a fuel injected engine. Are there some benefits to running with a carburettor that make up for the hassle of carb icing?
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I don't claim to understand the engineering behind the decision, but the WWII Rolls Royce Merlin engine had carburettors instead of FI as a deliberate design choice. They got more power from the engine with carbs.
It must have been pretty compelling. Until Miss Shilling developed her 'orifice', the engine cut out in inverted flight / negative G.
It must have been pretty compelling. Until Miss Shilling developed her 'orifice', the engine cut out in inverted flight / negative G.
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Based on my limited experience, the main difference is that injected engines require sacrifices to various deities, a fair amount of black magic and frequent flat batteries in order to achieve a start, whereas carby equipped ones go first time 
Being serious, advancements the like of which are very good for cars aren't necessarily applicable to aircraft. My take would be that while FI is a boon for part throttle operation, throttle response and fuel economy around those conditions, I'm not entirely convinced it would make that much difference for aircraft that tend to operate over a smaller RPM range/at constant RPM. Why increase complexity if it brings little benefit? Or to put it another way, the wheel is archaic technology, but we're still using them - they just work!
Is negative G capability really of a requirement for something like a Spitfire? I'd think finding yourself negative was probably a close precursor to being shot?
Being serious, advancements the like of which are very good for cars aren't necessarily applicable to aircraft. My take would be that while FI is a boon for part throttle operation, throttle response and fuel economy around those conditions, I'm not entirely convinced it would make that much difference for aircraft that tend to operate over a smaller RPM range/at constant RPM. Why increase complexity if it brings little benefit? Or to put it another way, the wheel is archaic technology, but we're still using them - they just work!
Is negative G capability really of a requirement for something like a Spitfire? I'd think finding yourself negative was probably a close precursor to being shot?
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The most important advantage of fuel injection over carburettors in a typical flat aircraft piston engine is ability to achieve similar air-to-fuel ratios in all cylinders. The problem with carbureted engine is that usually there is only one carburetor, so the fuel is mixed with air at the beginning of air intake system. The air-fuel mixture is then delievered to engine via intake ports, but usually there is less-than equal distribution of the mixed air between cylinders, which basically gives you four single-cylinder engines trying to work together, which can be quite interesting sometimes. The exception to this are some high performance engines, for example most cars designed for racing, which used carburetors, had one carburetor per cylinder. In most engines, this means that you have one cylinder which runs with excessive rich mixture, one with just right, one slightly lean and one which (when the engine is leaned) barely runs at all - the latter is usually reason for rough running of the engine, which then forces you to enrich the mixture and thus burn more fuel. In the cockpit, you see the difference of the air-to-fuel ratios in the cylinders by EGTs, which are more than 150°F (I wish I were kidding) apart in some installations, and it seems that it only gets worse by WOT (wide open throttle) operation, where the engine is normally the most efficient, due to the least airflow drag on the throttle plate/butterfly.
Of course, to outcome this problems, you need to match the air-fuel ratios in each cylinder, which TCM and Lycoming solved by putting fuel injectors at each cylinder's intake port and "match" the fuel flows to minimise the difference in mixture ratios - obviously they could do better or GAMI would be out of work
Not to confuse with modern car engine (be it diesel or gasoline), which has fuel injected directly in cylinder during intake stroke (even multiple times in single cycle in common rail engines), the aircraft engines usually use continuous flow fuel injection, where you basically only control the amount of fuel flow through the injector. This "complicated" technology has been available for decades, but I assume most manufacturers use carburettors for low-end (two/four seat training aircraft) because their design is simple, and it comes with a lot smaller price tag. I was very surprised to see Cessna put carbureted O-200 in their 162, since they do put IO-360 in latest C172 model. Even Diamond went with TCM IO-240 for their DA20-C1 and it proved as a quite good engine (except for some variants with altitude compensating fuel pump, but that's another story) for now.
As for troubles with starting hot/warm fuel injected engines, they are far less difficult to start if you know what happens in an engine during period between shutdown and startup, but carbureted are usually really a bit easier to start when warm. Reading the engine's Operational manual doesn't hurt either.
Of course, to outcome this problems, you need to match the air-fuel ratios in each cylinder, which TCM and Lycoming solved by putting fuel injectors at each cylinder's intake port and "match" the fuel flows to minimise the difference in mixture ratios - obviously they could do better or GAMI would be out of work
Not to confuse with modern car engine (be it diesel or gasoline), which has fuel injected directly in cylinder during intake stroke (even multiple times in single cycle in common rail engines), the aircraft engines usually use continuous flow fuel injection, where you basically only control the amount of fuel flow through the injector. This "complicated" technology has been available for decades, but I assume most manufacturers use carburettors for low-end (two/four seat training aircraft) because their design is simple, and it comes with a lot smaller price tag. I was very surprised to see Cessna put carbureted O-200 in their 162, since they do put IO-360 in latest C172 model. Even Diamond went with TCM IO-240 for their DA20-C1 and it proved as a quite good engine (except for some variants with altitude compensating fuel pump, but that's another story) for now.As for troubles with starting hot/warm fuel injected engines, they are far less difficult to start if you know what happens in an engine during period between shutdown and startup, but carbureted are usually really a bit easier to start when warm. Reading the engine's Operational manual doesn't hurt either.
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Is negative G capability really of a requirement for something like a Spitfire?
As I understand it, Miss Shilling's orifice was a plate with a hole that more or less kept the fuel where it was supposed to be in negative G.
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Actually the problems with fuel starvation on Merlins was overcome by moving the point where the fuel entered the intake much further downstream, a kind of pre-cursor to single point injection on a lot of today's modern cars.
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Aha, now there's a proper answer (flying stone)! Note that I was being slightly facetious about starting procedures - it just feels like that some days 
I would point out however that direct (cylinder) injection petrol engines are a (relatively) recent innovation, and even now, the bulk of car engines poke the fuel out into the inlet manifold upstream of the inlet valve - they are however generally not a continuous flow system (once had an old saab with an early mechanical FI that was). Potentially upstream is a better system for fuel mixing and generating maximum power outputs; I seem to remember seeing one (racing?) application where there were two sets of injectors - the second set being further upstream and coming online at high rpm. In any case, in-cylinder seems to permit better economy - they can play with when fuel is injected in the stroke, and apparently even the mixture ratio in different parts of the cylinder(!) to sustain very very lean burn, reduce pumping losses by throttling it on mixture rather than a butterfly (LOP anyone?), etc.
I would point out however that direct (cylinder) injection petrol engines are a (relatively) recent innovation, and even now, the bulk of car engines poke the fuel out into the inlet manifold upstream of the inlet valve - they are however generally not a continuous flow system (once had an old saab with an early mechanical FI that was). Potentially upstream is a better system for fuel mixing and generating maximum power outputs; I seem to remember seeing one (racing?) application where there were two sets of injectors - the second set being further upstream and coming online at high rpm. In any case, in-cylinder seems to permit better economy - they can play with when fuel is injected in the stroke, and apparently even the mixture ratio in different parts of the cylinder(!) to sustain very very lean burn, reduce pumping losses by throttling it on mixture rather than a butterfly (LOP anyone?), etc.
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The two advantages of injectors are:
More uniform fuel distribution between cylinders ie they all run at the same mixture and therefore all achieve peak EGT at approx the same setting.
This doesn't work as well with carbs so you need to run the weakest cylinder at the correct point and all the rest then run richer than optimum. Thus wasting fuel
Also injected engines don't suffer from carb icing. They will still ice up in extreme conditions but nowhere near as easily as carburetted engines.
The downside of fuel injection is the cost and the need for a fuel pump
More uniform fuel distribution between cylinders ie they all run at the same mixture and therefore all achieve peak EGT at approx the same setting.
This doesn't work as well with carbs so you need to run the weakest cylinder at the correct point and all the rest then run richer than optimum. Thus wasting fuel
Also injected engines don't suffer from carb icing. They will still ice up in extreme conditions but nowhere near as easily as carburetted engines.
The downside of fuel injection is the cost and the need for a fuel pump
Engineer speaking here. 
Further to Flying Stone's post re injectors, hot starts are quite easy once you know the technique (always worked for me in Africa with warm engines and OATs approaching 40 C).
Mixture - Cut-off.
Prime (Continental) or boost pump (Lycoming) until you have a steady fuel pressure reading. Pump off.
Throttle wide open.
Start! Be ready to retard the throttle and advance the mixture when the engine fires.
A result every time!
Further to Flying Stone's post re injectors, hot starts are quite easy once you know the technique (always worked for me in Africa with warm engines and OATs approaching 40 C).
Mixture - Cut-off.
Prime (Continental) or boost pump (Lycoming) until you have a steady fuel pressure reading. Pump off.
Throttle wide open.
Start! Be ready to retard the throttle and advance the mixture when the engine fires.
A result every time!
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Fuel injection is better on every front.
Some FI engines are a bit tricky to hot-start though. You have to follow the POH procedure, and you don't exactly want to have a half flat battery when doing it. That is about the only advantage of a carb.
I would never buy a plane with a carb. One doesn't need the excitement of carb icing. Irritating enough on a lawn mower.
Some FI engines are a bit tricky to hot-start though. You have to follow the POH procedure, and you don't exactly want to have a half flat battery when doing it. That is about the only advantage of a carb.
I would never buy a plane with a carb. One doesn't need the excitement of carb icing. Irritating enough on a lawn mower.
Would direct injection be of any benefit in GA engines, especially if you could increase the compression ratios by half a point as most car makers did when they switched from port injection do direct injenction..
Intersesting tho is that the Toyota 86/Sabaru BRZ has both port and direct injection.
Intersesting tho is that the Toyota 86/Sabaru BRZ has both port and direct injection.
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According to Sir Stanley Hooker one of the main benefits of of a carburetor in the Merlin as opposed to the direct fuel injection used by Daimler Benz in the Me 109 etc was the cooling effect of the fuel evaporation which offset the heating effect of the supercharger compression and resulted in a higher density of fuel/air mix reaching the cylinders = more power.
(They had to put an intercooler on the later more highly supercharged Merlins and when "fuel injection" was adopted I think it was still single point indirect injection upstream of the supercharger intake)
That argument isn't applicable to naturally aspirated engines where the cooling effect of the carburettor can (apparently) all too easily result in carb icing - which seems to be pointed to in all too many AAIB accident reports.
But so long as the light aircraft industry is stuck with 1920's engine designs, carb icing will continue to create work for the AAIB. While turbochargers and fuel injection is fitted to millions of cars.
(They had to put an intercooler on the later more highly supercharged Merlins and when "fuel injection" was adopted I think it was still single point indirect injection upstream of the supercharger intake)
That argument isn't applicable to naturally aspirated engines where the cooling effect of the carburettor can (apparently) all too easily result in carb icing - which seems to be pointed to in all too many AAIB accident reports.
But so long as the light aircraft industry is stuck with 1920's engine designs, carb icing will continue to create work for the AAIB. While turbochargers and fuel injection is fitted to millions of cars.
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As it's fuel injection under discussion and Gami got a mention here's a long, current thread running over in the Australian section of the site.
http://www.pprune.org/pacific-genera...nt-course.html
Rob
http://www.pprune.org/pacific-genera...nt-course.html
Rob
The big advantage of a carburated system is that when combined with a high wing aircraft there is typically no requirement for any fuel pump of any kind. Other than the obvious simplicity and maintenance issue it also reduces the fire hazard.
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I wonder how many cars are still being built with carburettors? Mind you, I used to know how to fix and tune an SU carb. Injection is just black magic but seems to give twice the performance on half the fuel burn.
Don't think the same progress has been made with GA engines!
Don't think the same progress has been made with GA engines!
Last edited by astir 8; 29th Apr 2013 at 21:03.
Originally Posted by nomorecatering
Intersesting tho is that the Toyota 86/Subaru BRZ has both port and direct injection.
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This is nicked directly from Wikipedia with all the usual caveats. Not direct injection straight into the cylinders
"Further improvements were introduced throughout the Merlin range: 1943 saw the introduction of a Bendix-Stromberg pressure carburettor that injected fuel at 5 pounds per square inch (34 kPa; 0.34 bar) through a nozzle directly into the supercharger, and was fitted to Merlin 66, 70, 76, 77 and 85 variants. The final development, which was fitted to the 100-series Merlins, was an S.U. injection carburettor that injected fuel into the supercharger using a fuel pump driven as a function of crankshaft speed and engine pressures.[45]"
"Further improvements were introduced throughout the Merlin range: 1943 saw the introduction of a Bendix-Stromberg pressure carburettor that injected fuel at 5 pounds per square inch (34 kPa; 0.34 bar) through a nozzle directly into the supercharger, and was fitted to Merlin 66, 70, 76, 77 and 85 variants. The final development, which was fitted to the 100-series Merlins, was an S.U. injection carburettor that injected fuel into the supercharger using a fuel pump driven as a function of crankshaft speed and engine pressures.[45]"
Last edited by astir 8; 30th Apr 2013 at 16:04.