How important are fuel temps?
I just saw photos of amemiyas new rx8 for d1. i was pretty surprised when i saw three separate fuel coolers in the trunk of the car! thought they were oil coolers but everything routes back to the swirl tank. could high fuel temps cause a loss of power?
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i found someone had said they had a 10% loss on the dyno because of high fuel temperatures. i live in arizona so this could be an issue for me. my car is noticably slower on a hot day.
does temperature have a similar effect as different octanes? lower temps would take longer to reach the flash point because they have a farther gap to close. i dont want to worry about detonation because of the weather out here.. maybe a fuel cooler wouldnt hurt imo. |
Maybe this would get more of a response in a different sub-forum?
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I know my tune definitely changes with hot fuel vs. cold fuel. I can tune my idle and drivability to be perfect with a just-warmed-up engine, but if I drive it for a long enough time for the fuel in the tank to get heat soaked, the drivability starts to fall apart and the idle goes to crap.
The OEMs have made a mad dash to returnless systems for good reason: keep the fuel cool instead of constantly circulating it to the hot engine and back. They mainly did it for emissions reasons, since hot fuel results in higher evaporative emissions. But think about that a moment... hot fuel evaporates easier, or to put another way, the fuel takes less heat out of the intake in the process of evaporating. If you are running a carb or fuel injectors shooting down from the end of an intake trumpet, you're getting clear power benefits from this heat of evaporation cooling the air down and making it denser... and if the fuel is hot, you lose those benefits... There's an amount of snake oil in cool cans, but there's some truth to their usefulness too. |
Originally Posted by peejay
(Post 10666191)
They mainly did it for emissions reasons, since hot fuel results in higher evaporative emissions. But think about that a moment... hot fuel evaporates easier, or to put another way, the fuel takes less heat out of the intake in the process of evaporating. I think i understand what your talking about. but i dont think its the process of evaporating that causes it. fuel has to sit in an open space for a long while to evaporate. but it wont stay there much longer though once its been ignited! your explanation makes me think about all those horrible emissions components and about trying to lower hydrocarbons. LOL but i know thats not where you were going with it. and i appreciate the reply, im thinking this thread should be moved to the rotary performance section.. maybe it would get more input. and i really want a straight answer on this. If we were able to monitor the temperature of the air/fuel mixture before its been ignited, im sure we would be able to plot out the increase in power along with the decrease of fuel temps. but the closest we can come to that is monitoring IAT's. which is recorded before fuel is mixed. Someone correct me if im wrong, but i think since fuel is much denser than air, it would have the most effect on overall temps. Like Hot fuel would greatly increase the air temps. We spend so much time making sure our air charge is icy cool, but if were spraying heat soaked & uncooled fuel into it then temps still go up in the end. I guess thats why egt's are important during tuning? It sounds like it would help to make sure the car isnt just at operating temperature before a tune, but also that fuel temps are somewhere near daily driving conditions. another reason why i prefer street tuning over dyno. |
Originally Posted by jplecas
(Post 10673003)
I think i understand what your talking about. but i dont think its the process of evaporating that causes it. fuel has to sit in an open space for a long while to evaporate. but it wont stay there much longer though once its been ignited!
Originally Posted by jplecas
(Post 10673003)
Someone correct me if im wrong, but i think since fuel is much denser than air, it would have the most effect on overall temps. Like Hot fuel would greatly increase the air temps. We spend so much time making sure our air charge is icy cool, but if were spraying heat soaked & uncooled fuel into it then temps still go up in the end. I guess thats why egt's are important during tuning?
The heat energy required to increase the temperature of the gasoline to the boiling point is much lower than the amount required to perform the actual phase change (vaporization), but a change in fuel temperature would have a minor effect on the intake charge temperature. The amount of heat required to raise the temperature of 1g of gasoline by 1º C is 2.22 Joules/g-C (heat capacity) http://www.engineeringtoolbox.com/sp...ids-d_151.html. But the latent heat of vaporization of gasoline is 348.9 J/g. http://www.afdc.energy.gov/afdc/pdfs/fueltable.pdf (150 btu/lb converted to J/g) The boiling point of gasoline varies depending on ethanol content, etc..., so I'll just use 100ºC for the example. So for the energy required to raise 1g of 90ºC gasoline to 100ºC: q = C_p * T = 2.22 J/g-C * 10ºC = 22.2 J/g Add the 348.9 J/g for vaporization, and you get 371.1 J/g of total energy used If you had a lower starting temperature, say 70ºC, and increased it to 100ºC, you would have: q = 2.22 J/g-C * 30ºC = 66.6 J/g 66.6 J/g + 348.9 J/g for vaporization = 415.5 J/g total energy So that 20ºC starting temperature difference results in an energy change of about 12% (415.5 J / 371.1 J). If we assume a stoichiometric air/fuel ratio (14.7:1), you have 1g of gasoline for every 14.7g of air. The energy needed to heat and vaporize the fuel is extracted from the air, and in this case it gets split 14.7 different ways. The heat capacity of air at atmospheric pressure & near the temperature range in question is 1.009 J/g-C (http://www.engineeringtoolbox.com/ai...ies-d_156.html). So for example 1: Delta_T (change in air temperature) = q / C_p / 14.7 Delta_T = (371.1 J/g) / (1.009 J/g-C) / 14.7 = 25ºC For example 2: Delta_T = (415.5 J/g) / (1.009 J/g-C) / 14.7 = 28ºC So the temperature drop of the intake charge after gasoline is injected and vaporized changes by only 3ºC. So for every 1ºC the gasoline temperature increases, the intake charge temperature will increase by 0.15ºC. That's not really much at all. This is pretty simplified, but I think it still gives a good overview. Hopefully I didn't botch the math somewhere :icon_tup:. |
Excellent info! Sounds about right intuitively speaking. It would seem there isn't much to be gained from a performance perspective, but I suppose every little bit helps.
As far as oems, the money they would save on material by not having to run fuel lines back to the tank would probably be enough for them to justify return-less fuel systems, performance benifits aside lol. |
Originally Posted by sharingan 19
(Post 10682529)
As far as oems, the money they would save on material by not having to run fuel lines back to the tank would probably be enough for them to justify return-less fuel systems, performance benifits aside lol.
Looking at the math, *wow* the heat of evaporation is a lot compared to simple cooling. I didn't realize it was that intense for gasoline. Great, now I am thinking about fuel *heaters*. But only the fuel that stays near the engine. Hmm, another use for a returnless fuel system. |
The heat of vaporization is a much bigger deal on gasoline direct injection. You know Mazda's new Sky engines run 14:1 compression on 87 octane? That's because the fuel is injected directly into the intake during the compression stroke at about 100bar of fuel pressure, as opposed to around 3 bar on multiport injection. They also have a factory long runner 4-2-1 header.
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you guys should see this research paper Mazda did back in the 90s on prototype direct injected rotaries.
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direction injection would make a world of diffrence on those of us wishing to run 87 octane in a high compression turbo engine. to bad there's so much $$$ involved in the parts themselves, i'm sure we could mill and drill some housings to accept them...
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Originally Posted by arghx
(Post 10683131)
you guys should see this research paper Mazda did back in the 90s on prototype direct injected rotaries.
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3 Attachment(s)
Here are some diagrams from the old prototypes, which were pre-Renesis peripheral exhaust port engines with a strange "subchamber" ignition system.
https://www.rx7club.com/attachment.p...1&d=1309237307 https://www.rx7club.com/attachment.p...1&d=1309237307 Mazda has been playing around with injector locations in their prototypes for years and years now. A few years ago they patented a GDI rotary engine with three injectors. One was the "upstream" design. That's the primary injector. The second injector sprays inside the engine during the intake stroke. The third (auxiliary, tertiary, whatever you want to call it) is in the conventional intake port location. Staging is based on rpm and load. Here is a sketch of the two injectors located on the rotor housing: https://www.rx7club.com/attachment.p...1&d=1309237519 The diagrams seem to show a lower cost solenoid-type injector that you would see in say a Ford GDI application. This is as opposed to the piezoelectric injectors used on common rail diesels and favored by BMW for GDI applications. |
^ You know I've been really curious as to what pressures Mazda is running on the new above injector placements? I wouldn't think you would need as much since it's NOT in FULL compression. All this time I was thinking that the injector pointing towards the intake was the primary. I guess I had that backwards. LOL!
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Of course these are still prototypes but I'm sure it's probably some slightly modified off-the-shelf injector system they got from a supplier (Denso most likely). So the fuel pressure probably varies between 20 and 100 bar like every other system.
If you look at the control strategies for GDI engines, fuel pressure usually has more to do with RPM and load than the injection timing. On piston engines they don't always fire during the compression stroke. Your basic "homogenous" style GDI engine, running lambda = 1 (not a lean burn/stratified engine) fires once during the compression stroke normally. However during warm up they will use various "split injection" strategies with two events per cycle in an effort to get the cat hot. Engines with more responsive injectors (piezoelectric instead of solenoid type) may have 3 injection events even during normal operation. That's how BMW does it on their homogenous engines. |
I'm not sure exactly how this translates to street cars, but If you know anyone who races motocross seriously, they all chill their fuel. I was told that it is universally known as the easiest way to add (or save) hp.
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Originally Posted by JK64
(Post 10683733)
I'm not sure exactly how this translates to street cars, but If you know anyone who races motocross seriously, they all chill their fuel. I was told that it is universally known as the easiest way to add (or save) hp.
i think this points out that what ever fuel you run might have a temp range that it works best at. |
Originally Posted by peejay
(Post 10683068)
Looking at the math, *wow* the heat of evaporation is a lot compared to simple cooling. I didn't realize it was that intense for gasoline
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Originally Posted by arghx
(Post 10683130)
The heat of vaporization is a much bigger deal on gasoline direct injection. You know Mazda's new Sky engines run 14:1 compression on 87 octane? That's because the fuel is injected directly into the intake during the compression stroke at about 100bar of fuel pressure, as opposed to around 3 bar on multiport injection. They also have a factory long runner 4-2-1 header.
VW's direct injection engines are 13:1 N/A and 11:1 with 14psi on top, but they may require 91 minimum. |
Originally Posted by arghx
(Post 10683341)
The diagrams seem to show a lower cost solenoid-type injector that you would see in say a Ford GDI application. This is as opposed to the piezoelectric injectors used on common rail diesels and favored by BMW for GDI applications.
And that they went for the top tier of injector tolerances. No programming the PCM to a new injector's characteristics like GM. |
The heat of vaporization of gasoline is pitiful when you compare it to water or methanol. Water is 2257 J/g and methanol is 1104 J/g. That's where the most substantial gains from aux. injection come from. Of course with water injection, an excess of water can interfere with the vaporization of the fuel being injected and lead to negative effects on power/efficiency. All the more reason to get the injection amount right.
Originally Posted by arghx
(Post 10683130)
The heat of vaporization is a much bigger deal on gasoline direct injection. You know Mazda's new Sky engines run 14:1 compression on 87 octane? That's because the fuel is injected directly into the intake during the compression stroke at about 100bar of fuel pressure, as opposed to around 3 bar on multiport injection. They also have a factory long runner 4-2-1 header.
But it also discusses the problems direct injection has with cold start emissions due to the reduced amount of time between fuel injection, ignition/combustion & the exhaust stroke. Especially in turbo applications because of the heat loss from the turbocharger. One of the solutions mentioned is to retard ignition timing after a cold start to increase the amount of heat in the exhaust system. The facts/rumors about the future 16X engine have me very interested in how efficient the rotary can become. Direct injection and laser ignition sound like huge advances for the rotary.
Originally Posted by Liborek
(Post 10684292)
Assuming complete evaporation. Unfortunatelly, many people and designs can´t utilize this fully... and choose to blame gasoline as bad fuel against miracle blends of today:blush::lol:
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5 Attachment(s)
Originally Posted by peejay
(Post 10684302)
The Ford trainer told us that the Ford direct injection engines were piezo.
Originally Posted by RotaryRocket88
But it also discusses the problems direct injection has with cold start emissions due to the reduced amount of time between fuel injection, ignition/combustion & the exhaust stroke. Especially in turbo applications because of the heat loss from the turbocharger. One of the solutions mentioned is to retard ignition timing after a cold start to increase the amount of heat in the exhaust system.
Finally the plug fires and the very hot mixture goes right out the exhaust valve, past the turbo and into the cat. On a port injected engine you can't do this as well because you have to spray the injectors a lot earlier. But then again, on port injected engines that kind of strategy isn't usually needed in the first place. |
One "dirty" little secret about DI engines is that the intake manifolds and ports, being completely dry of fuel, are bad to clog up with carbon. Even in an extremely close mounted port injection scheme where the injectors actually run into the cylinder head, there's enough runner wetting to keep things relatively clean.
Heck, I notice this on my port injected engines which all have had only two injectors. The primary runners stay clean, the secondaries get carbony, and when I was running the Turbo II manifold the secondary side plenum would get standing oil in it! And this was with engines that had no crankcase ventilation into the inlet tract. The Subaru TGVs are used on port injected engines to speed up airflow past the injectors. I'm unfamiliar with the VWAG implementation... |
The lack of fuel spraying on the valves does present a challenge with carbon buildup but they're not all bad. The VWAG engines certainly have certainly developed a reputation for being the worst with carbon buildup though. The BMW engines haven't been out as long but they certainly don't have a reputation for widespread carbon buildup problems. But they also have pretty sophisticated air-oil separation for the PCV system. Injection timing, injector design and location, etc.
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Originally Posted by Liborek
(Post 10684292)
Assuming complete evaporation. Unfortunatelly, many people and designs can´t utilize this fully... and choose to blame gasoline as bad fuel against miracle blends of today:blush::lol:
here is a system that heats and vaporizes the fuel. http://vfis.us/ The more surface area that is available to mix with air, the less unburnt fuel and energy loss we get. So if a higher fuel temp vaporizes more air, it results in a more efficient combustion. That article says HC's are ELIMINATED!?OMGWTF!? because they can vaporize the fuel 100% with the use of a heated vaporization chamber. once the gasoline is in a completely gaseous state, then its mixed with air and moved into the combustion chamber. All of the gas gets used, emissions are reduced, power is increased.. if thats all it takes why havent they been using it? I ended up reading that this is how a lot of those 200+ mpg claims were made in the 70s. and i found a huge list of patents of fuel vaporizers too. http://www.rexresearch.com/ogle/1ogle.htm
Originally Posted by RotaryRocket88
(Post 10684871)
Originally Posted by Liborek
(Post 10684292)
Assuming complete evaporation. Unfortunatelly, many people and designs can´t utilize this fully...
But for the purposes of this discussion, the fraction of fuel that never evaporates isn't important for comparison of fuel temperatures.
Originally Posted by peejay
(Post 10683068)
Great, now I am thinking about fuel *heaters*.
It looks like the way these guys are doing it is by placing the vap chamber directly above the exhaust manifold. Or using an electric heater. That article also mentioned using hot air from the radiator. |
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