Naked Science Forum
Non Life Sciences => Chemistry => Topic started by: Hasan on 27/11/2010 14:30:03
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Hasan asked the Naked Scientists:
What is more efficient as a vehicle fuel, petrol or diesel?
What do you think?
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In an engine optimized for diesel, diesel fuel is more efficient than petrol. In an engine optimized for petrol, petrol is more efficient than diesel.
Oh! You mean something like, "Given equal amounts of fuel, which could go farther, a truck with a diesel engine or a truck with a petrol engine?"
Well, consider that petrol engines vary greatly in their efficiency. Some small petrol-burning passenger vehicles can get 40 to 50 miles per gallon while I've read that large trucks are doing well if they can get 5 miles per gallon. I don't know what the range of fuel efficiency is for diesel engines. I imagine there would be some overlap.
Wikipedia says that diesel has a slightly higher energy content than petrol -- 38.6 megajoules/liter for diesel, 34.8 for petrol. So I think that means diesel has the potential to be more efficient, but I think the efficiency determined in any particular test would depend on how well the engines used were designed and tuned. I think you could find diesel engines that are more efficient than some petrol engines and vice versa.
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Wikipedia says that diesel has a slightly higher energy content than petrol -- 38.6 megajoules/liter for diesel, 34.8 for petrol.
True, but by weight, gasoline has a slightly greater energy. We really ought to buy fuel by weight anyway. When we buy by volume, we are getting less energy for our money when a liquid fuel is warm.
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I remember in the early 70's as the gas prices increased diesel prices were low and stable.
There were very few automobiles running this fuel.
Efficiency was out of this world, until the trend of the prices affected diesel too.
The price adjustment between diesel and petrol has compensated the dollar per mile between each fuel.
Comparing gallon per mile diesel wins hands down, diesel gets better mileage.
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Hasan asked the Naked Scientists:
What is more efficient as a vehicle fuel, petrol or diesel?
What do you think?
It doesn't depend on the fuel only, as others wrote. Diesel engines (with diesel fuel, of course) are, generally, more efficient because of some reasons, the main of which is the greater compression ratio, which gives greater thermodynamic efficiency.
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Diesel burns at a lower temperature than petrol and hence is more efficient from a thermodynamic perspective. This means that less fuel does more work and hence diesel tends to perform better in terms of efficiency.
That said, diesel engines tend to be much heavier and also warm up less quickly, so the real efficiencies are only seen when the engines run to temperature on a long journey.
Chris
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Diesel burns at a lower temperature than petrol and hence is more efficient from a thermodynamic perspective. This means that less fuel does more work and hence diesel tends to perform better in terms of efficiency.
That said, diesel engines tend to be much heavier and also warm up less quickly, so the real efficiencies are only seen when the engines run to temperature on a long journey.
Chris
Chris, I thought that there was less fuel to air mixture using diesel.
Also, Diesel fuel has less ignition vapors than petrol, I can die out a match in a can of diesel but not in a can filled with petrol???
The flashpoint of a fuel is the lowest temperature at which it can form an ignitable mix with air. The high flash point in diesel fuel means that it does not burn as easily as gasoline, which is a safety factor. Too low of a flash point is a fire hazard because ignition may continue and lead to explosion
http://hypertextbook.com/facts/2005/EileenTang.shtml
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Diesel burns at a lower temperature than petrol and hence is more efficient from a thermodynamic perspective. This means that less fuel does more work and hence diesel tends to perform better in terms of efficiency.
From the Carnot heat engine perspective, the opposite is true.
The simple answer is, it depends!
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Diesel burns at a lower temperature than petrol and hence is more efficient from a thermodynamic perspective.
Er, no [;D]
Diesel engines tend to be more thermally efficient (meaning they convert more chemical energy into work) for two main reasons.
1. As Lightarrow points out, they operate at higher compression ratios. This means that the fuel air mixture burns at a higher temperature. Consequently, more work can be extracted during expansion of the hot exhaust. This is achieved by having a greater expansion ratio. (The expansion ratio is the same as the compression ratio.)
2. Diesel engines don't have a throttle to limit the mass of air inhaled during the intake stroke. This "free breathing" characteristic reduces pumping losses compared with spark ignition engines. Diesels can do this because they don't need to mix fuel and air in the right proportions for combustion. Instead, the desired amount of fuel is simply injected into the hot air that was produced during the compression stroke. The power output is determined by the amount of fuel injected as opposed to the mass of air inhaled which is the case with spark ignition engines.
Recently however, the distinctions between diesels and spark ignition gasoline engines have become a bit blurred due to the introduction of direct injection gasoline engines and sophisticated valve control methods that effectively increase the expansion ratios of gasoline engines.
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Most diesels are turbo-charged these days, Geezer, so actually what was a pre-load (in the form of intake restriction) is now after-load (prop in the exhaust).
C
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Most diesels are turbo-charged these days, Geezer, so actually what was a pre-load (in the form of intake restriction) is now after-load (prop in the exhaust).
C
Chris,
The point is that diesel engines have lower pumping losses than spark ignition engines. Turbocharging, whether applied to diesel engines or spark ignition engines has little effect on the thermal efficiency of those engines. Turbocharging does allow a relatively smaller displacement engine to generate a greater amount of power compared to a normally aspirated engine. This helps to reduce the mass of the vehicle which can have an effect on overall efficiency, but that applies to both diesel and spark ignition engines.
Geezer
UPDATE:
I should have mentioned that turbocharging is more prevalent with diesel engines because they typically produce less power than a gasoline engine of the same displacement. The main reason for this is because a gasoline engine has a greater RPM range, so, if required, it can convert more chemical energy into work than is possible with a diesel. A turbocharger on a diesel has the effect of increasing the apparent displacement.