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how does fuel temperature effect performance/economy?
by
NintendoKD
on 23 Jul, 2012 09:08
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I have a question, since the idea is to make the fuel and air mix more completely, then wouldn't it be a better idea to have fuel at or near the same temperature of the compressed air in the cyl? this way heat doesn't have to exchange before proper vaporization occurs. I mean this seems like a no brainer but can I get some engineer input here?
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#1
by
R.O.R-2.0
on 23 Jul, 2012 11:51
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I have a question, since the idea is to make the fuel and air mix more completely, then wouldn't it be a better idea to have fuel at or near the same temperature of the compressed air in the cyl? this way heat doesn't have to exchange before proper vaporization occurs. I mean this seems like a no brainer but can I get some engineer input here?
intake air temp is a variable.. how are you going to heat/cool the fuel fast enough for any of this to matter?
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#2
by
ORCoaster
on 23 Jul, 2012 12:11
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Cool air is generally better for engines as it is more dense than hot air. Thus you have more oxygen in it. Similar to sea level compared to mountain air.
But all compression heats air so how will you match it in the cylinder, and the temp there is different at least at start up.
Heating fuel can be tricky, veg oil needs heat to flow and burn like diesel, diesel heats at the injectors and cools the IP. Gas, we all have gas but remember the days of gas heating on a summer day in a carb? Youngsters won't but vapor lock sucks in downtown traffic jams. Running down to the local McDs for a cup of ice to pack on the engine to start up again was a pain then and doesn't happen now with FI.
Best to not fool with the way things work. And what works is leave fuel alone and cool the air coming in, IC for you?
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#3
by
nathantheengineer
on 23 Jul, 2012 13:28
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not to mention the safety aspect either, i wouldn't want to drive around with a tank of fuel close to the flash point!! after all the return will heat the fuel in the tank over a longish journey.
As for carb icing i had a mk1 ford escort with twin weber 40's, more than 20 mins on the motorway on a cold night then they were weber 6's with all the ice build up
happy days!!
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#4
by
carrizog60
on 23 Jul, 2012 13:50
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modern tdis and such,have this fuel intercooler device,dont know if the same theory applys to idi
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#5
by
wolf_walker
on 23 Jul, 2012 14:27
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I can tell you a sufficiently worn injection pump is QUITE sensitive to both fuel and ambient air temp.
As was said over a longish journey the fuel is going to end up being about the temp of the IP near as I've ever been able to tell
with an IR thermometer. And that pump in the winter can be quite chilly when cruising on the highway with good airflow.
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#6
by
NintendoKD
on 23 Jul, 2012 23:08
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The object is to properly mix the diesel and air to create a more complete combustion right? I was thinking along the lines of vaporizing the diesel pre-injection, or getting close to it. This way the diesel doesn't have to absorb the energy from the compressed air to properly mix, and the compressed air doesn't have to shed heat/energy to give to the fuel to make a proper mix. this process takes time, and makes the diesel want to re-condense, as opposed to vaporize, "think, how does a de-humidifier work?" like the extreme difference in potential energy. I have an idea to do this, and I do realize that having tepid diesel/fuel go through the IP is a bad idea I want to heat the injector lines after the pump. the pump gains better pressures from increased viscosity=cooler fuel right? so returning fuel is not a lot here, I was thinking cool the fuel post filter pre pump, the heated fuel can't go back while heating expanding because the pump acts as a one way valve. The heating is a two step process to prevent the heat from coming back to the pump, but essentially there isn't anything special about my setup save the fuel cooler. I picked up a few from LAV upgrades when they threw the old ones away. The fuel expands after the pump, creating even more pressure the only drawback I see is there would be some kind of warm up valve adjustment to retard the timing to prevent the injectors from popping too soon. You would have to retard it anyway the fuel would burn...... well....... too well you would make more power and better fuel economy, plus lower emissions. just spitballin here.
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#7
by
NintendoKD
on 23 Jul, 2012 23:29
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I understand that there is "more" oxygen in the air when it is cooler cool air on cold mornings with a cold start = peppier car, starting to notice the trend here??? The common factor here is not so much air volume "it does to a small degree" but air temp, the air and fuel are at a close approximate temp, therefore less energy is lost but engines are not designed to run like this because they are heat pumps and get hot so the air gets hot while the fuel stays "relatively" cold the bigger the difference the more inefficient it is. How much "more" oxygen are we talking??? the idea is to pack energy into the air, and to "allow" the release of the potential energy in the fuel by the use of infared/electromagnetic means. The carbon acts as a transducer to varying degrees. the less energy that has to be absorbed by the carbon the more energy it can release, but an equilibrium of kinds must be reached. example we tell our guys to drink warm water in hot weather because it absorbs into the body faster, why? because the temperature has to equalize first. the closer the value energy/temp is to one another the easier it is to release the stored potential energy, to a certain degree. I can't have an ice cold diesel start on ice cold diesel, but a 100f deg air temp + 100 deg fuel temp is much better for combustion, which I imagine increases as temperature goes up. I imagine that it keeps going up as temp of both increases to a certain degree. This is the very basis that rudolph had in mind for the diesel engine, as I understand it. The way our motors are designed will not immediately work with this concept in mind, there will need to be a mechanical control of some kind that increases and decreases pump control/timing with temperature of air and fuel, and I'm not talking about tdi here, I really believe that IDI is the way to go. key to understanding more of this is IONIZATION, not COMBUSTION, also understanding that the ICE is only two things, an air pump, and a heat pump.
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#8
by
nathantheengineer
on 24 Jul, 2012 00:29
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Unfortunately it is not the fact that the temps of the fuel and air on a cold morning are approximately the same that increases the 'peppiness' of your car.
It is a simple thermodynamic law that density increases with cooler temperatures. It is the density of the air that increases the 'pep' of your car. Think of it simply as the cold (and dense) air has more calorific value.
Rather than heat the lines why dont you increase the pressure of the pump? Higher presssure= higher temperature.
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#9
by
745 turbogreasel
on 24 Jul, 2012 00:43
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So those guys running meth injection must be losing mad power?
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#10
by
gldgti
on 24 Jul, 2012 04:56
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important not to confuse 2 things here.
intake air temperature (say, in the manifold) is going to be anywhere between 10-110 deg C, depending on how hot the engine is, the weather, the boost, etc etc etc.
But, the temperature of the air in the combustion chamber before the fuel is injected is VERY HOT, regardless of how warm it is before it is compressed by the piston.
The OP's suggestion is to heat the fuel to closer to the temperature of the air in the cylinder, right before injection. This temperature is ~210 deg C (anywhere between say 180 and 300 more like).
If you could figure out a way to heat the fuel closer to this temperature, then i guess in theory, upon injection, it will absorb less energy from the air charge and the peak pressure will be higher.
But, if you consider the actual thermal mass of an injection charge - it is very small. Infact, it is much smaller than the thermal mass of the air charge.
The heat capacity of diesel is about 1750 J / kg·K. an injection charge for a VW diesel making about 75kW is about 6 mm^3. 6mm^3 of diesel weighs around about 5.1e-3g, or 5.1e-6kg From thermodynamics, we know that:
deltaH = m*c*deltaT, that is the change in heat energy is equal to the mass x heat capacity x temperature difference. So if we run the numbers to find out how much energy it takes to heat that much diesel fuel from say, 60C to 210C (deltaT will be 150K), we have
deltaH = 5.1e-6 x 1750 x 150
= 1.33875 Joules.
Now, if the engine is making 75kW, then the air mass if the AFR is pretty rich for power making and boost is around 25psi and the engine speed is say 3000rpm is about 1.065e-3kg.
The heat capacity of air at ~210C is about 1026 J/kg.K. So, the intake charge has, at 210C:
H = m.c.T
= 1.065e-3 x 1026 x (210+274)
= 528.86 J of heat energy.
Now, remember that this air mass is calculated with an AFR of 16:1, pretty rich for a diesel, which normally might run under full power 19 or 20:1 AFR, and at idle more like 200:1 - so even in this very rich condition, the amount of thermal energy needed from the air charge to bring the diesel fuel up to combustion temperature, is only
(1.33875/528.86)*100 = 0.25% of the thermal energy of the air charge. Not much really.
btw - I am not being a smart ass - I was actually curious about your idea so I have sat here in the cold for about 30minutes preparing this response for all to see, please take it kindly, in the spirit is has been given :-)
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#11
by
NintendoKD
on 24 Jul, 2012 07:21
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So those guys running meth injection must be losing mad power?
I know right? But they do make more power, that is why I am asking all of the questions here, seems a bit confusing, and I'm sure I'm not the only one.
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#12
by
NintendoKD
on 24 Jul, 2012 07:24
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important not to confuse 2 things here.
intake air temperature (say, in the manifold) is going to be anywhere between 10-110 deg C, depending on how hot the engine is, the weather, the boost, etc etc etc.
But, the temperature of the air in the combustion chamber before the fuel is injected is VERY HOT, regardless of how warm it is before it is compressed by the piston.
The OP's suggestion is to heat the fuel to closer to the temperature of the air in the cylinder, right before injection. This temperature is ~210 deg C (anywhere between say 180 and 300 more like).
If you could figure out a way to heat the fuel closer to this temperature, then i guess in theory, upon injection, it will absorb less energy from the air charge and the peak pressure will be higher.
But, if you consider the actual thermal mass of an injection charge - it is very small. Infact, it is much smaller than the thermal mass of the air charge.
The heat capacity of diesel is about 1750 J / kg·K. an injection charge for a VW diesel making about 75kW is about 6 mm^3. 6mm^3 of diesel weighs around about 5.1e-3g, or 5.1e-6kg From thermodynamics, we know that:
deltaH = m*c*deltaT, that is the change in heat energy is equal to the mass x heat capacity x temperature difference. So if we run the numbers to find out how much energy it takes to heat that much diesel fuel from say, 60C to 210C (deltaT will be 150K), we have
deltaH = 5.1e-6 x 1750 x 150
= 1.33875 Joules.
Now, if the engine is making 75kW, then the air mass if the AFR is pretty rich for power making and boost is around 25psi and the engine speed is say 3000rpm is about 1.065e-3kg.
The heat capacity of air at ~210C is about 1026 J/kg.K. So, the intake charge has, at 210C:
H = m.c.T
= 1.065e-3 x 1026 x (210+274)
= 528.86 J of heat energy.
Now, remember that this air mass is calculated with an AFR of 16:1, pretty rich for a diesel, which normally might run under full power 19 or 20:1 AFR, and at idle more like 200:1 - so even in this very rich condition, the amount of thermal energy needed from the air charge to bring the diesel fuel up to combustion temperature, is only
(1.33875/528.86)*100 = 0.25% of the thermal energy of the air charge. Not much really.
btw - I am not being a smart ass - I was actually curious about your idea so I have sat here in the cold for about 30minutes preparing this response for all to see, please take it kindly, in the spirit is has been given :-)
An engineer answered my question, thanks, just made my day. Where did you glean this information?
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#13
by
ORCoaster
on 24 Jul, 2012 14:29
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He's an engineer. He went to school or is in school and learned the principles and the formulas and he is now calcualting a resulant answer based on your hypothetical potsualtions.
It is what they do. How do I know? 1 gd, 1 dad, and three brothers just like him. Needless to say family gatherings are generally not spent discussing such stuff but given enough fuel aka alcohol, the conversation can drift in this direction. Until one of the wife's pokes her head in and cusses us out.
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#14
by
gldgti
on 24 Jul, 2012 21:22
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Where did you glean this information?
the calcs are mine, but I did cheat some:
I used the not2fast website turbo calculator to get my "amount of air" starting figures.
And, I searched around for a guideline figure for the injection quantity. Found that 6mm^3 on tdiclub. I would say its about in the right order of magnitude.
Yes, engineers and family gatherings.... my sister hates it, she says all I do is talk about "science". She did a BA in "film and stage"
