Heating value and thermal efficiency
Discussion
Just wondering if anyone here may know how the heating value of a fuel used in an engine fits in with the thermal efficiency.
For instance, diesel engines can run on a mix of LNG and Diesel. In fact they can be made to run almost purely on LNG with a little diesel used to start the ignition (LNG has a higher autoignition temp that diesel so song normally start combustion just under compression alone, although it can and this is treated like knock in a SI engine).
However, thermal efficiency normally drops.
Now I have read you can increase the thermal efficiency by advancing the "ignition" (the time at which you use some diesel to get things going) and you can use a higher Cetane diesel.
The LNG typically has a Octane of 120 + (can be quite high) and an autoignition of around 600 deg (diesel is 250-350). This, coupled with the typically lean burn conditions mean longer ignition delay, and longer combustion duration (except when LNG % is much higher than % diesel), cooler burning and lower cylinder pressures.
Now those 3 things I guess would be the reason why thermal efficiency would go down, the slower burn is effectively working the piston less even though LNG has a higher lower heating value than diesel (50 MJ/kg vs 43 MJ/kg, i.e. there is more energy available if you combust LNG completely). That is why the above things help. Advancing means it gets to exert force on the piston earlier and higher cetane pilot fuel obviously speeds up the ignition/combustion creating the same effect.
I guess as well, due to lean burn conditions for the LNG, it doesn't burn completely (high unburned hydrocarbon emissions are a problem with these engines) so you may not be reaching that lower heating value anyway.
But I just have trouble seeing how the heating value fits. Or does it? Is it the heating value that causes the higher auto ignition temps or the higher octane numbers? Or is it just that the HV is the HV and the reason why efficiency drops simply due to the fact that given the lean conditions it is burned under, it takes longer to burn, burns cooler and incompletely = less energy, over longer time = less energy extracted by the piston.
For instance, diesel engines can run on a mix of LNG and Diesel. In fact they can be made to run almost purely on LNG with a little diesel used to start the ignition (LNG has a higher autoignition temp that diesel so song normally start combustion just under compression alone, although it can and this is treated like knock in a SI engine).
However, thermal efficiency normally drops.
Now I have read you can increase the thermal efficiency by advancing the "ignition" (the time at which you use some diesel to get things going) and you can use a higher Cetane diesel.
The LNG typically has a Octane of 120 + (can be quite high) and an autoignition of around 600 deg (diesel is 250-350). This, coupled with the typically lean burn conditions mean longer ignition delay, and longer combustion duration (except when LNG % is much higher than % diesel), cooler burning and lower cylinder pressures.
Now those 3 things I guess would be the reason why thermal efficiency would go down, the slower burn is effectively working the piston less even though LNG has a higher lower heating value than diesel (50 MJ/kg vs 43 MJ/kg, i.e. there is more energy available if you combust LNG completely). That is why the above things help. Advancing means it gets to exert force on the piston earlier and higher cetane pilot fuel obviously speeds up the ignition/combustion creating the same effect.
I guess as well, due to lean burn conditions for the LNG, it doesn't burn completely (high unburned hydrocarbon emissions are a problem with these engines) so you may not be reaching that lower heating value anyway.
But I just have trouble seeing how the heating value fits. Or does it? Is it the heating value that causes the higher auto ignition temps or the higher octane numbers? Or is it just that the HV is the HV and the reason why efficiency drops simply due to the fact that given the lean conditions it is burned under, it takes longer to burn, burns cooler and incompletely = less energy, over longer time = less energy extracted by the piston.
Otispunkmeyer said:
Or is it just that the HV is the HV and the reason why efficiency drops simply due to the fact that given the lean conditions it is burned under, it takes longer to burn, burns cooler and incompletely = less energy, over longer time = less energy extracted by the piston.
I think the HV is just the HV. The heating value tends to have more of an effect on the initial combustion temperature. There is maybe some correlation between it and autoignition and cetane numbers due to bond energy and chemistry things I'm not too hot on, but thermal efficiency is much more to do with how quick and completely the fuel burns. Another big part as well is heat loss - the longer the fuel takes to burn the more time it has to lose that energy to the walls rather than using it to do work. I think what I'm trying to get at is thermal efficiency is as much to do with engine design and combustion circumstances as it is to do with HV. What I've also seen is that the heat release per mol doesn't really change with mixture - lean or rich (at least in what I've been looking at which is SI engines).
For example you could have a very low HV with little energy released overall but if most of that energy is put to use then the thermal efficiency could still be reasonable.
Sorry if this doesn't make much sense - my first post in the science forum and I'm a little braindead at this time of night.
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