can you please explain. I cannot see any relationship between compression ratio and fuel consumption.
cimmex
Merlin vs. DB601 (2 Viewers)
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cimmex
wuzak
Captain
Higher CR means less boost required from the compressor, thus less power expended in driving the compressor, thus less fuel used for the same (or more) power output.
Sounds reasonable but I don't buy it. It would be far easier to increase the CR than improve the charger to get more boost. I wonder why RR didn't go that way.
cimmex
Shortround6
Major General
Actually higher compression means a higher expansion ratio in the cylinder. More work is extracted from the fuel burn before the exhaust valve opens and dumps the remaining pressure to atmosphere. However you only have so many BTUs of energy in the fuel/air mixture in the cylinder. The supercharger crams more fuel/air into the cylinder so you have more BTUs to begin with. More power even if a bit less efficient. Rounding off a Merlin at 12lb boost (27lbs total) compared to one at 6lbs boost (21lbs total) was cramming 28% more fuel and air into the cylinders. You need a LOT of compression to get 28% more power.
Superchargers heat the intake mixture more than just the simple heat of compression unless they are 100% efficient (never happens) and a hot mixture is less dense even at the same pressure (less mass). A better supercharger needs a bit less power to drive, heats the air less for the same amount of boost.
Superchargers heat the intake mixture more than just the simple heat of compression unless they are 100% efficient (never happens) and a hot mixture is less dense even at the same pressure (less mass). A better supercharger needs a bit less power to drive, heats the air less for the same amount of boost.
The inverted V requirement dates back to a committee of 1928, made up of various German experts. They decided that future liquid cooled aero engines should incorporate the following principal design features.
- mono-block cylinder banks,
- wet cylinder liners,
- propeller reduction gear,
- supercharger,
- fuel injection,
- high temperature glycol cooling,
- provision of a cannon tunnel in the V
These requirements were presented to the manufacturers as a fait accompli. Tender documents were sent to Daimler-Benz, Junkers and B.M.W., all of which eventually produced a V-12 engine model in response although none was able to incorporate all of the required features immediately. The DB600 had dry liners and carburettors, the JU210 also had only carburettors and the B.M.W. 116/117 only got as far as the prototype stage.
Wolfram Eisenlohr, in 1928 head of the DVL power plant division and a member of that committee, was interviewed in 1980 about the 1928 requirement for inverted V-12s and he cited three reasons for the decision.
- more compact installation,
- better pilot view for single engine aircraft,
- less exhaust flame dazzle during night flying.
The manufacturers were much less keen to turn their engines upside down.
A report was made by a British team which visited Germany immediately after the war, interviewing many of their German counterparts. The Report is called, "Comments on Visit to Germany, July 24th 1945 to August 12th 1945".
10 persons made up the visiting party, including P. G. Barlow and E. Wolsey (M.A.P.), Mr A. Thomas (Armstrong Siddeley Motors), Mr N. Quinn (Bristol Aero Co.), Mr R. Chamberlin (D. Napier Son) and Mr G. Morris (Rolls-Royce Ltd).
Page 3 of the Report says,
"A good example of Air Ministry [RLM] control lies in the inverted Daimler-Benz engine. The D.B. people said that both from a technical and production point of view they would have preferred to make an upright engine but they were compelled to make it inverted by the Air Ministry."
Later.
"With the inverted engine, they [engineers from DB] said it was very difficult to obtain consistent oil consumptions and due to the rotation of the crankshaft, one bank gets more oil than the other. For this reason the engine is built with a lower compression ratio on one bank than the other."
So it seems inversion also caused problems that the German engineers would rather not have had to deal with.
I don't think that the minimal peripheral advantages of such an installation ultimately justified the engineering challenges it raised, with the possible exception of the cannon tunnel.
Cheers
Steve
SR6 explained that well.
RR was of opposite opinion - they 1st improved Merlin's supercharger with Merlin XX in 1940, then introduced two-stage supercharger with Merlin 60, already in second half of 1941 in token numbers.
Sorry, cannot see any hint in SR's post that higher CR lowers the fuel consumption.
cimmex
I've done the same, but if you modify a engine to make more power, and you use that additional power, then of course your fuel mileage goes down.
But I noticed WHEN I drove the modified higher compression engine for mileage, ( which wasn't often) it was more efficient and got better mileage.
Aircraft engines were designed from ground-up for certain CR, the CR being there in most designer's equations from day one. Nobody skimmed the heads in some depot hundreds or thousands miles away from factory. Plus, increased CR will reduce the allowable boost pressure, so more would be lost than gained.
The influence between CR and consumption was mentioned several times in 'Vee's for victory', where Allison went with no less than 6.65:1 when designing the V-1710; lower CR than that was deemed to increase the consumption. Allison was envisioning the V-1710 as air-ship and bomber engine, where consumption is a major thing. Later/post war, consumption was sidetracked, the CR went to 6:1 with G series of engines, so the boost could be increased and the engine output be closer to the same generation of Merlins.
There might be no way for people skimming heads to note decrease of consumption - they were eager to use up the newly-acquired power, consumption was not their aim. We do not now, however, the specific consumption (ie. consumption per HP produced).
The influence between CR and consumption was mentioned several times in 'Vee's for victory', where Allison went with no less than 6.65:1 when designing the V-1710; lower CR than that was deemed to increase the consumption. Allison was envisioning the V-1710 as air-ship and bomber engine, where consumption is a major thing. Later/post war, consumption was sidetracked, the CR went to 6:1 with G series of engines, so the boost could be increased and the engine output be closer to the same generation of Merlins.
There might be no way for people skimming heads to note decrease of consumption - they were eager to use up the newly-acquired power, consumption was not their aim. We do not now, however, the specific consumption (ie. consumption per HP produced).
fastmongrel
1st Sergeant
You cant just increase compression ratio you need new cam profiles, new valves, you need to retime the magnetos and design a new exhaust and inlet system. You cant cram as much fuel air mix in so you have to lower supercharger ratio or the tops of the pistons and the valve heads will disintegrate. Also more compression and you have to start strengthening parts as there aint no such thing as a free lunch.
My forum name Fastmongrel comes from a bike I used to race an Aermacchi 350cc air cooled pushrod single, a mongrel mix of several different bikes. It was fast for an old thumper but it could be blown away by a japanese 250 2 stroke twin, I decided it was time for more speed I fitted a bigger carb, bigger exhaust, bigger valves, lumpier camshaft, new crank, piston and rod and skimmed the head to raise CR. After all this work it went faster in a straight line but the power had been shoved too high up the rev range and it had no mid range power, it actually went slower round a twisty circuit it had lost the flexibility. I ended up taking it all back to original apart from the piston, rod and crank.
My forum name Fastmongrel comes from a bike I used to race an Aermacchi 350cc air cooled pushrod single, a mongrel mix of several different bikes. It was fast for an old thumper but it could be blown away by a japanese 250 2 stroke twin, I decided it was time for more speed I fitted a bigger carb, bigger exhaust, bigger valves, lumpier camshaft, new crank, piston and rod and skimmed the head to raise CR. After all this work it went faster in a straight line but the power had been shoved too high up the rev range and it had no mid range power, it actually went slower round a twisty circuit it had lost the flexibility. I ended up taking it all back to original apart from the piston, rod and crank.
Wiking85,
An add on to this would be service life and maintainability. Which had better longevity and which were more easily repaired.
I remember reading an article in, I think Air and Space, about the various WW2 fighter motors. The author made points that the German engines (DB and Jumo-213) both had 1/3 less parts than the RR Merlin. More parts means more chances for failure. I think he also went on to mention that the Allison could be overhauled in the field, while the Merlin, DB and Jumo could not.
Cheers,
Biff
davebender
1st Lieutenant
Modern aircraft and tank engines are designed to be quickly replaced in the field. Removed engine is shipped back to factory or rear area depot for rebuild. If WWII era U.S. Army rebuilt engines in a field environment then our operational doctrine was behind the times.
That's not quite correct; while the ideal situation is to have plug-in components, in wartime conditions it might not be possible to depend on rear-echelon services being available when or where they were needed - ground crews might be forced to work with what is available in front of them because there are no replacements. During WW 2 there were numerous examples of supposedly unserviceable aircraft and engines being made serviceable.
Packard, for instance, recognised this and issued a V-1650-3 7 Maintenance Manual with this revision:
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davebender
1st Lieutenant
Your reference is for emergency repair and replacement. Everyone does that. It's not the same as an engine overhaul.
Shortround6
Major General
Sometimes authors get misled by simple numbers. I have no doubt that a R-R Merlin had more total "parts" than some other engines but many of these "parts" were screws and bolts used to hold thing together or on the engine. Rolls-Royce, even on their cars, liked to use more small screws/bolts spaced closer together to hold on things like cam covers than many other manufacturers. Lets face it, with a V-12 engine there are only 12 pistons now matter who makes it, there is only one crankshaft and so on. Things were you can get more parts are in the valve train. Merlins used 4 valves per cylinder but so did some other V-12 engines, triple vs double valve springs? A more complicated oil pump or more of them? German fuel injection used way more parts than the British carburetor.
DB roller bearing connecting rods used 72 rollers per pair of connecting rods and so on.
With out a meaningful break down of the number of critical or even just "moving" parts the "total" number of parts actually doesn't tell us much about the reliability of an engine although it may tell us a bit about how hard it was to overhaul. Having to deal with a significantly higher number of fasteners could be a pain in the butt even if they make no real difference to the life of the engine.
There are any number of text books about engine design that go into compression ratio. Many from the 1930s and 40s can be found online for $10-20 ALL of the ones I have seen say that higher compression will give better gas mileage or fuel consumption. In some case they will give the result of tests done on test stands with the engine connected to a dyno. Sometimes they have charts/graphs showing the theoretical increase in power by changing the compression ratio with everything else staying the same. Theory and formulas were sometimes backed up by experiments like ones for friction and pumping losses in which a test engine was spun by an electric motor with various parts removed to find out what each part or group of parts contributed to the total friction. Bare crankshafts were spun in the main bearings, "L" head cylinder head was removed to measure crankshaft and piston/ring friction without compression. Power to run fan and so on.
I think I will take their word for what was going on inside some of these engines.
DB roller bearing connecting rods used 72 rollers per pair of connecting rods and so on.
With out a meaningful break down of the number of critical or even just "moving" parts the "total" number of parts actually doesn't tell us much about the reliability of an engine although it may tell us a bit about how hard it was to overhaul. Having to deal with a significantly higher number of fasteners could be a pain in the butt even if they make no real difference to the life of the engine.
There are any number of text books about engine design that go into compression ratio. Many from the 1930s and 40s can be found online for $10-20 ALL of the ones I have seen say that higher compression will give better gas mileage or fuel consumption. In some case they will give the result of tests done on test stands with the engine connected to a dyno. Sometimes they have charts/graphs showing the theoretical increase in power by changing the compression ratio with everything else staying the same. Theory and formulas were sometimes backed up by experiments like ones for friction and pumping losses in which a test engine was spun by an electric motor with various parts removed to find out what each part or group of parts contributed to the total friction. Bare crankshafts were spun in the main bearings, "L" head cylinder head was removed to measure crankshaft and piston/ring friction without compression. Power to run fan and so on.
I think I will take their word for what was going on inside some of these engines.
Small changes in the design of critical components can have a big influence on how well an engine performs in service. For example, the Merlin needed several small modifications to enable it to run at +18 lbs boost; these changes led to improvements which enabled higher power outputs:
View attachment Lovesey on Merlin extract.pdf
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