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#1
06-23-2011, 07:18 PM
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#2
06-23-2011, 07:25 PM
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Quite.
To a lesser degree the same argument applies to British engines, given that the most successful airliner airframes were American in 1945. Therefore comparison between the Merlin and the R-2800, R-3350 or R-4360 in the civil market isn't really fair; it makes more sense to compare it with the Bristol Hercules or Centaurus, and if you perform that comparison then the Merlin doesn't look quite so much of a "failure" in the civil market anymore...
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#3
06-23-2011, 08:44 PM
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Both Daimler-Benz and BMW were forbidden from even being in the aviation market. Post-war, both companies withdrew from anything to do with aviation and produced automobile engines instead. Both are industry leaders from the moment they entered the market and that leadership continues today. They produced some of the best engines in the world. Quote:
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#4
06-23-2011, 09:01 PM
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In fact in the 1950's, we started doing it.....
In the R-4360C Wasp Major power-plant with CH 9 turbo-blower..... Quote:
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#5
06-23-2011, 10:27 PM
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Charge temperature is limiting, and you'd obviously rather get your supercharger work from the turbine than the crankshaft. So you throw away the supercharger, but that means you need to either go to DI or else inject into the eye of the turbosupercharger impeller to homogenise the mixture. The turbocharger came from GE, whilst the piston engine came from P&W. Fuel injection into the turbocharger wasn't viable because of the fire risk, both in case of leaks between the hot and cold sides of the turbocharger, and because of the relatively long ducting from turbocharger to piston engine, which would otherwise have been full of stoichiometric mixture. But most importantly, it wasn't viable because it would have been almost impossible to start the engine unless the turbocharger was clutched to the crankshaft for that purpose, which in turn wasn't possible due to the physical separation between turbocharger and piston engine which was itself a consequence of the historical decision that GE would make turbochargers in isolation from the piston engine manufacturers. The thermodynamic benefit comes from utilisation of exhaust enthalpy which would otherwise have gone to waste. However, there is an enthalpy loss equal to the sum of enthalpy drop across the aftercooler, and the cooling drag on the cold side thereof; if fuel had been injected upstream of the turbosupercharger, the compression process would have had a higher apparent isentropic efficiency, and the aftercooler would have had less work to do because the compressor delivery temperature would have been lower. In essence, the benefit comes from improved matching/work balance rather than from going to DI itself. In other words, they wanted to throw away the supercharger to get more of their compressor work from the turbocharger, and this drove them to DI because they then didn't have a method to homogenise the mixture. So DI is a consequence rather than a cause.
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#6
06-23-2011, 11:50 PM
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That's an interesting take on direct injection, Viper.
I'd always assumed DI was first introduced to more reliably control mixture, eliminate pre-ignition and get a stratified charge in CI systems.
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#7
06-24-2011, 01:31 AM
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In general, a single carburettor isn't likely to give good mixture distribution to a multi-cylinder naturally aspirated piston engine, because evaporation isn't completed before the charge reaches the intake manifold, and so you get a fairly complex multi-phase flow. However, if you've got a supercharger between the carburettor and the intake manifold, things get a lot better because the supercharger homogenises the mixture and also increases its static temperature. This means that much more, if not all of, the fuel evaporates; diffusion is then very helpful in further homogenising the mixture. Furthermore, the flow is likely to warm the induction manifold sufficiently that fuel doesn't condense upon contact with it. This means that supercharged engines which put the fuel into the airflow (whether via a carburettor or some kind of injection system) upstream of the supercharger will tend to deliver a pretty consistent FAR to all of their cylinders. This removes one of the main motivations for direct injection. However, it must be stressed that this is something of a special case; take the supercharger away and FAR will vary considerably from cylinder to cylinder, whereas with DI you can set FAR quite accurately. As an aside, WWII vintage technology would just meter the fuel into the cylinders, which is an open-loop approach. You'd still get FAR variations from cylinder to cylinder because although the fuel mass injected would be the same for all cylinders, the airflow would not. A modern car engine would use an oxygen sensor to tune the fuel mass injected into the cylinder so as to maintain stoichiometry throughout the operating range of the engine; this is vital to the operation of 3-way catalysts. However, this sort of closed-loop approach requires computers, and it is primarily driven by emissions legislation rather than engine performance (power, SFC) considerations. Without such constraints you'd run the engine leaner to improve SFC, or richer to improve power. Fuel injection is very useful for CI engines because injection timing can be controlled in order to control the timing of the combustion event; it also allows the pressure profile of the combustion event to be controlled. Limiting the peak cylinder pressure allows you to make the engine lighter. With modern engines you can also just stop fuelling cylinders in order to reduce power. This is useful because the turn-down ratio of the injectors is limited if you want good atomisation; poor atomisation leads to reduced combustion efficiency and increased emissions (especially CO and UHC). The alternative is to use multiple injectors per cylinder, but that's a pain. (If you really really want to then you can build a CI engine with a carburettor, but it's hard work, and it tends to be difficult to control combustion in a satisfactory manner, which hurts thermal efficiency and will tend to cause vibration due to considerable cycle-to-cycle variations in engine behaviour. You'll also find that the smoke limit is set by the richest cylinder, and smoke is a factor then this inevitably limits output.) Anyway, DI is great for mixture control, but as with all aspects of engine design, it's an option within a tradespace, rather than an unmitigated upside. If you're mostly interested in operating at a fixed design point then the advantages of DI may easily be outweighed by other options, especially if you're supercharging heavily. OTOH, if you're making a small engine for an economical passenger car today, it's very hard to beat a turbocharged CI engine with DI. So I'm not suggesting that carburettors are magic; what I'm saying is that DI isn't magic either. Which is better depends upon your priorities, and the job you're trying to make your engine do. In the very specific example of R-4360C, the primary goal was to get rid of the supercharger so that more useful work could be extracted from the exhaust via the turbocharger. The removal of the supercharger then drove the design towards DI. But it's not reasonable to say that DI is thermodynamically superior; the advantage comes from the improved utilisation of exhaust enthalpy, and DI is just a tool which allows this to be done. Indeed, had it been possible to inject fuel upstream of the turbosupercharger's impeller, superior performance would have been attained. So DI is analogous to a bridge in this case; the economic benefit comes from the traffic which the bridge carries, rather than the bridge itself, and life would have been easier & cheaper if there had been no need to build a bridge in the first place. But I must stress again that I'm talking about big aero-engines with high degrees of supercharge here; very different conclusions would be reached if the engine in question was designed to power a car, or even a significantly smaller aeroplane.
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#8
06-24-2011, 02:45 AM
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Have you ever flow an piston engine aircraft with individual EGT/CHT? The CHT and EGT will be vastly different if the fuel metering system is not direct injection. Each cylinder is being meter a different amount of fuel. That means power loss just in the thermal differences! Not to mention that none of them are a stoichiometric mixture. Add to that, it is impossible to optimize the timing advance. All of your cylinders are developing different power levels and none of them are optimal. There is no why to precisely control how much fuel goes to each cylinder in an intake manifold. With direct injection, you can not only optimize timing advance to the power curve, you can maintain a stoichiometric mixture. The thermal losses are eliminated because your CHT/EGT's are the same. Quote:
A simple illustration of that basic principle. 1006 J/kgC 460 J/kgC 2100 J/kgC To change the temperature of a mass of 1 Kg of each by 2 degrees…. Air = 1006 J/kgC * 1kg* 2 C = 2012J Fuel = 2100J/kgC*1kg*2 C = 4200J Steel = 460J/kgC * 1kg * 2 C = 920J Our 4200J of fuel energy goes to cool the 15C air… 4200J * 1kg /1006J/kgC = Change in T = 4.17 C 15C - 4.17C = 10.83C Now let us dump our fuel on the hot steel of our combustion chamber. 4200J * 1kg / 420J/kgC = Change in T = 10 C 15C - 10C = 5C 5 degrees Celsius is much colder than 10 degrees Celsius. Last edited by Crumpp; 06-24-2011 at 02:59 AM.
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#9
06-23-2011, 09:48 PM
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However, Daimler has quite a big stake in EADS, whilst BMW started a joint venture with RR to make turbofans in Germany from 1990, though now this is 100% owned by RR. Quote:
A supercharger is a pretty effective way to homogenise a mixture. The intake manifold is going to end up at roughly charge temperature, which for a Merlin at high power is going to be about 90ºC. You are very unlikely to see condensation of the fuel onto the manifold at that temperature. FAR will therefore be pretty constant from one end of the manifold to the other. Charge distribution may well vary, which would modify CHT somewhat, but the same argument applies to air distribution. FAR will become variable when supercharger delivery temperature is low, and this will affect acceleration behaviour, especially from low boost & revs. But aero-engines spend most of their time at fixed, relatively high, power settings, and so this sort of transient behaviour is far less of a problem for an aero-engine than for a car engine. Quote:
The supercharger is basically adiabatic if you're not injecting fuel or water into it. However, isentropic efficiency of superchargers tends to be much lower than the isentropic efficiency of the compression stroke of a piston engine. In any case, you're always going to gain more by reducing temperature as early in the compression process as possible, because compressors (whether steady-flow or non-flow) produce temperature ratios in exchange for pressure ratios, whilst the absolute work required for the compression process is proportional to deltaH, i.e. Cp*deltaT. If you reduce the starting temperature then you reduce the deltaT all the way down the chain, and the benefit multiplies. Therefore, if your fuel is liquid, you really want to inject it at or before the start of the compression process in order to maximise the thermodynamic benefit associated with its latent heat of evaporation. Clearly for a naturally aspirated engine you might as well go for direct injection, especially if the number of cylinders is small. The cylinders & pistons are very far from being adiabatic, but are very efficient at performing compression work. The limiting factor is the rate at which they can pass non-dimensional flow through their intake & exhaust valves at any given rpm. Hence supercharging; pre-compressing the air allows you to get more absolute mass flow rate into the fixed non-dimensional mass flow capacity of the piston engine. That's the objective of the exercise. You use a steady flow machine upstream of the unsteady flow machine because unsteady flow machines are inherently bigger than steady flow machines, and therefore you can shrink the physical size of the engine in relation to its effective flow capacity. Quote:
EGT and CHT will be different anyway because that's life; holding FAR constant is great but it's not magic; airflow into the cylinder depends upon induction manifold design and engine speed. Induction manifold design is quite a complex business, and compromises are inevitable. DI is very useful if you want to vary non-dimensional power setting over a wide range, but this isn't so important for an aero-engine, and so the higher design-point efficiency offered by injecting into the eye of the supercharger is a pretty compelling argument, before you even consider the cost, mass and complexity advantages. Modern GA engines are going DI because they're going CI (in order to burn Jet-A and save money), and also because they don't have a lot of cylinders, which means that the cost of injectors is inherently less important.
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