by ffkiwi » Sat May 15, 2004 12:30 am
This is a bit of silly question and keeps cropping up! To put it in perspective
what is more powerful-a 200HP Ferrari or a 200HP John Deere tractor?
Next question-which one of these would you want to pull you out of a swamp? Pure BHP is quite misleading taken out of context-you need to know the intended use-propellor size, and a number of other factors.
Diesels, size for size, will always turn a bigger prop that the equivalent displacement glow-and as larger props are more efficient than smaller ones (hence the various geared glow engines and belt drives which have appeared over the years) so a model may well perform better on a diesel turning a larger prop than on the same size glow turning a somewhat smaller one-but this does NOT mean the diesel is more powerful, simply that its torque is better on larger props, and occurs lower in the rpm scale-and that automatically limits the BHP compared with an engine where the maximum torque occurs at a higher rpm. remember BHP is a derived function, not an actual one-all dynamometers (at least in model engine sizes) measure torque, and the torque figures in conjunction with rpm, generate the BHP curbe.
Despite their fixed ignition points, glows can and do rev higher than diesels-assuming their porting allows it-and as a consequence generally produce greater BHP figures for quivalent sizes-which is not necessarily the same as "power"-which tends to be misused by modellers when they should be talking about "thrust" as it directly relates to flying models-not academic BHP figures.
Again even this is a simplistic explanation, since 'power' needs to be defined in context-there are some competition classes in which the engine size is defined, and absolute power is paramount-C/L speed, F1C and pylon come to mind, others where 'power to weight' ratio is a factor-and a lightweight less powerful engine may be a better option than a heavier powerful one.
There is only one event in which the diesel is totally dominant-FAI team racing, and in that the diesel's dominance is due to fuel economy, not absolute power. Fuel economy is the diesel's one true advantage over glow engines-BUT-this can be overstated, since except for team race-and a few vintage events, fuel economy is NOT a factor in most competition classes-and frequently overlooked is the fact that diesel fuel is considerably more expensive than glow fuel, so converting glows to diesel will not necessarily save you anything in fuel costs-especially since the conversions tend to to be fairly expensive to start with. What a diesel conversion of a glow will do is:
(1) potentially allow you to turn a larger prop at the same revs as the glow
(2) allow you to turn a much larger prop than the same size glow at USABLE flying revs
(3) fly for longer on the same size tank OR
(4) fly for the same time on a smaller size tank
Apart from the initial cost of the conversion, and the intrinsically higher fuel costs, there are other hidden pitfalls-the operating stresses of diesels are significantly higher than for glows-and rods and shafts can and do fail.
Others on this forum have already mentioned the smell of diesel which is certainly stronger and more lingering than glow-but failed to mention that diesel exhaust-whilst oily, is less harsh on painted surfaces than glow-but paradoxically, many iron on coverings don't like it.
So rather than asking a question which cannot be directly answered, perhaps you should have defined the parameters better-or asked what advantages disadvantages there are for diesls vs glows
Is the query purely academic? In which case there is no accurate answer
-or if you were considering getting/using some converted glows-what were you planning to use them in?
From my personal 38 years modelling experience and from someone who owns over 300 engines-half of them diesel-the answers would be something along the lines of:
sport F/F- diesel or glow at personal preference
scale-diesel wins hands down for prop size and mild fuel effects
R/c sport-glow operation is easier
fuel alotment events-diesel again
pure HP determined events-racing glow
vintage events-diesel in preference to ignition (where permitted)
There are other factors to be considered as well-diesels rarely throttle as well as glows-and converted glows often create problems-which can often be solved by fitting a SMALLER carb. The fuel is highly flammable and far less widely available than glow fuel-and there is no equivalent component in diesel fuel that can boost power the way nitromethane does in glow fuels.
Diesels in general do not like pressurised fuel systems-which can pose problems in some setups-and particularly important in F/F and limited ER R/C events- do not cut as cleanly as glow engines and tend to run on, resulting in overruns.
So essentially the answer to all this is: define your requirement, and then you can decide whether a diesel is appropriate