View Full Version : Fuel rate ? Pedal vs actual
DA BIG ONE asked about fuel enrichment and I did not want to hijack his thread but I have alway wondered about the fuel rate actual vs commanded and diesel fuel rate in general.
As I understand it diesel's are fuel regulated. Add fuel and rpm goes up no throttle body and engine just pumps air the more the better. Simple concept but I seem to feel a difference whether its loaded or unloaded. More than just boost I think. Also, I am guessing there is some kind of govenor mechanism ( but not a rev limiter on the 6.5) as in most cases the 6.5 won't blow up if I floor it w/o load. I suspect there is some kind of feedback or mechanism for difference in actual fuel rate vs throttle position potential fuel rate so whats going on there?
Does the accelerator pedal actually set a probable fuel rate and also some kind govenor mechanism and the IP pumps fuel to satisfy that govenored rpm (ignores air etc) and achieves it or tries to. But won't excessively exceed it when unloaded. Because I don't think it sets an exact fuel rate per pedal position as a big load differance would cause a big difference in actual engine rpm and that doesn't seem to happen. I can set my accelerator position and it either will go to about that rpm or require more fuel for the load but if no load on crankshaft it doesn't seem to runaway too many rpm's very easily.
First, without going into thermodynamics, what produces power in an engine (internal or external combustion) is the conversion of heat energy into mechanical energy.
In the internal combustion engine, this heat is extracted as the piston travels downward in its power stroke (per Boyle's Law, as a gas expands, it cools, and thus heat is extracted). There are many internal combustion "cycles", including Otto (typical gaso car engine), Miller (actually used on a very few Mazda Millenia), and soon to debut on some Subarus. Diesels, not surprisingly, use the Diesel Cycle.
There are no complex computations nor governor necessary for a diesel to function just fine. Some engines do use computers to refine the fuel economy, but that's an enhancement, not a necessity. Some do use governors, but that's for other reasons than to make the engine run. However, most diesels, regardless of computer "enhancements" depend on dumping more fuel into the cylinders to produce more power. Very simple.
The Otto Cycle depends on fuels burning at a certain rate; higher compression ratios require fuels to burn more slowly (octane rating), as combustion is initiated by a spark plug, and gaso engines are much more critical with respect to the fuel-air mixture. Gasoline burning too quickly results in knock. You may recall "accelerator pumps" on carburetted gaso engines; this dumped more fuel into the carburetor throat when the "skinny pedal" was mashed. This did two things - gave the engine an extra shot of fuel until the vacuum in the carb could suck more fuel into the venturi, and it momentarily richened the mixture, slowing the combustion down a bit.
Diesels are a whole "different smoke" (sorry, I had to do that), and are far less dependent on the fuel-air mixture for combustion. Diesel fuels' cetane rating is an expression of the time delay from injection (recall these are compression-ignition engines) to ignition. More fuel results in more heat, so more power, and the engine will speed up, and ingest more air, allowing more fuel to burn (more heat). Naturally-aspirated (i.e., non-turbo) engines, as you suspected, draw air into the combustion chamber through pressure differential (vacuum). Supercharged engines (whether mechanical superchargers or turbo superchargers) provide more air to be forced into the combustion process, thus more fuel can be burned, and more heat produced - and more power extracted from that heat. With turbos, the waste heat increases, the manifold pressure increases (more air, and hence more fuel can be burned - as turbos also extract heat and spool up if there's more to be had).
Contemporary diesels like the Cat C-series and the Cummins IS-series run turbo boost in the 20-30" range, allowing much more power to be extracted than if the engines were naturally-aspirated.
When I romp on my truck in 2nd gear it will rev up to about 3400 rpm and acts like it runs out of fuel and dances on a govenor. It won't do it in a higher gear quite so easily. I have revved it to near redline w/o any load and it does it with less "throttle". Now put a load on it and depends on throttle position and load as to what rpm it will run up to. But I feel like it pulls harder when its working like its burning more fuel towing than not towing even at the same throttle position. But speed is slower when loaded and burning more fuel.
With problems I have worked on small engines and poured a little fuel in cylinder and cranked it and it runs away to some rpm then runs out of fuel then cuts off.
It doesn't take much fuel to run up the rpms unloaded or light load. I can see the piston "running away" before all the fuel burns and wonder if something like that happens and there is some mechanism that helps prevent it. It may be so but its not intuitive to me that the IP will always inject all the fuel commanded and let the rpms run away if overfuelled for load. I guess I am forgetting the injection event is driven by the rpm of IP (tied to crank) and thats what I am forgetting to consider why it doesn't just run away but still seems there is a difference in percieved fuel rate delivery for differing loads. Was wondering if there was any feedback mechanism.
Similar ever notice going down a hill when it starts to coasts faster than engine is pushing the combustion sound goes away like it quits injecting fuel. ?? Or is that the piston running away from the expanding gases only and its still injecting the same amount of fuel and just wasting it????
DmaxMaverick
02-17-2008, 22:56
It will defuel at governed speed. It isn't a "switch", like on gassers, which is why it's called "defuel", and not "shut off". It will lessen the fuel delivery as it nears the governed speed, until it balances the load with the RPM. It will not "run away", like you describe. There is one fuel event for one combustion event, and no "extra" fuel for the next event. This is assuming the system is operating properly. Any excess fuel, after the combustion event, is depleted by the exhaust gas heat, which is what causes the black smoke. Black smoke is fuel burned (not combusted--converted to soot) with too little O2. The exact process that makes charcoal out of wood.
It will defuel at governed speed. It isn't a "switch", like on gassers, which is why it's called "defuel", and not "shut off". It will lessen the fuel delivery as it nears the governed speed, until it balances the load with the RPM.
Yes thats kind of what I think really the throttle position also determines an IP "governed" speed not necessarily just a set fuel rate. So that it only injects enough fuel to meet the load demand (if it can achieve this speed). Maybe I am just quibbling over symantics but I always hear people say the throttle position controls fuel rate but yes and no actually depends on load somewhat too.
Just for instance newer farm tractors have "electronic" speed controlled PTO's which maybe is similar to cruise control. The PCM sets an output shaft speed and ups the fuel delivery to better match a differering load but set speed again similar to cruise control not setting a fuel rate. This is not the same as setting a "governed" IP speed and setting a fuel rate per IP stroke exactly. Setting a fuel rate and "governed IP speed" actually wastes some fuel for light load. Maybe just another reason newer diesels get better milage common rail better meters the fuel independant of IP RPM.
Maybe I should say it like this. Is it that on a 6.5 the throttle position sets a max fuel delivery per IP stroke and some governed speed. And it will inject some range of fuel per IP stroke speed based on load. IF engine runs faster for light load fuel delivery is actually less than max and if load requires max fuel it is delivered per throttle position and IP stroke speed and engine speed is developed to what its capable depending on all the factors.
DmaxMaverick
02-18-2008, 11:57
Um...No.
The TPS determines the fuel volume. It is only "governed" at the RPM limits, either idle or max. It will not govern or compensate in the mid range. Your TPS input will be a set, determined volume delivery. It will not compensate for load, only RPM, which translates to load at idle and max RPM. The electronic PTO speed control you reference is much like a "cruise control", but there is no such feature during footfeed operation. A specific TPS input will translate to a specific fuel delivery, but the actual number depends on your PCM and other parameters, and can change from engine to engine, according to how it is calibrated, the temps, fuel viscosity, timing, etc. The governing only involves the PCM's struggle to maintain a set RPM at idle or max engine speed.
OK I am getting closer to fully understanding. I did forget the connection and importance of TPS and volume as it relates to IP rpm and injection event being tied directly to IP rotation and hence crank rotation. Keep the info coming I am still thinking about how it actually controls precise fuel metering.
Say at 30% TPS PCM commands FSD for say maybe about 20 mm^3 per 1000 strokes of IP. Well at 1600 crank rpms its only going to inject so much but at 2200 rpm its going to be injecting more just because the IP is pumping faster. So actual fuel delivery is more determined by TPS and RPM. RPM achieved will be related to load due to physics but not any governed setting until max RPM parameters are pushed.
Remember I am a manual transmission .... What happens when I am applying 5-10% TPS, 2000 rpm at crank and start down a steep hill and engine starts to compression brake (same TPS position) instead of pushing the truck does the fuel rate change or is it still injecting fuel at the prescribed TPS volume per stroke???? Does the sound of combustion change because the piston is overrunning the cylinder pressure increase or does the IP defuel since the engine is starting to "run away" kinda sorta? I thought I read the newer engines will cut off fuel injections during compression braking events and inject no fuel going down bigger hills. Was wondering if all diesel do this to some degree??? Or do older diesels just waste fuel going down hill unless you take foot off TPS and or disconnect crank to tires?
DmaxMaverick
02-18-2008, 14:03
It's all about fuel energy vs. friction. If going downhill with any TPS input, it will fuel at that level until it hits the governed max speed. At that time, it will defuel as necessary to prevent the engine from exceeding the speed, and will stop fuel volume above that. The TPS input makes no difference at that time. If your friction is more than the hill will "pull" the vehicle down, then your TPS input will increase or hold the engine/vehicle speed at higher speed than with no TPS input. This is because you are adding fuel energy to overcome friction. There will be a terminal velocity with any object, in this case a vehicle traveling downhill. Friction can be the powertrain (compression braking, actual mechanical component friction, etc.), tire contact with the road surface, wind drag, etc. The fuel rate will remain the same, according to the TPS input. If it were to increase fuel rate according to RPM alone (at a given TPS input), then you would have a runaway engine, to some degree. The only load would be the friction of the engine and connected powertrain or accessory components. The mechanical pumps do this to some degree, but the electronics offset the effect electronically. This is why the idle speed must be governed, and getting an unloaded midrange RPM to stabilize is so sensitive with Diesels. For example, it is near impossible to hold a Mercedes D series to hold a steady RPM between the min/max governed speeds with no load on the engine (early models were really bad, later not as bad). It will either run up, or down, depending on which side your foot input is according to the governor. The DB pumps are similar, but not nearly as profound. It could be the nature of an axial pump vs inline, the pressure control circuit, or just the design tolerances involved. I haven't really looked too far into that.
Are you going somewhere with this? Or are you just killing time with a curiosity? Good discussion, though. I'd like to get other input on it.
I guess curiousity mostly as I like to fully understand things and later while I am mulling things over other things might click together better for me.
Thats kind of my point with holding a mid range rpm just how does it do it. Because just setting a fuel volume delivery didnt' intuitively sound like it would hold a stable rpm operation to me without some kind of "stabilization or govenor" feedback.
Kinda sorta related...
I am wonder what causes the fishbite is it feedback in a control loop error to the PCM or IP or something kinda sorta similar. I have read the electronic noise just messes with the precise fuel metering and the result is a fishbite feeling. Also what causes the occasional "spring rap buck" just off idle 2nd gear when the springs S wrap and sends a surge of forward motion unwrapping. The PCM and fuel metering can get wacky and surge then compression brake the engine (its mostly so close to idle control that its probably related but was wondering about it).
DmaxMaverick
02-18-2008, 15:29
Fishbite is an error in the PCM signal, mechanical deficiency, or fuel volume interruption. Caused by either a flaw not engineered out, or a failed/failing/malfunctioning component.
The reason we can get a reasonably stable mid range RPM is the fuel energy is offset by the engine friction. The "load", however slight, if you will.
The problem with the spring wrap surging is almost always caused by the driver's foot, surging the pedal in cadence with the surging springs. That's why you have to get all the way in or out of the pedal to stop it once it starts. It depends on just how bad it is at the time. The slow surge with a heavy load when accelerating from a stop is a combination of the above, the cargo load inertial surge, and the drivetrain cushion (spring-like, through all the components) loading and unloading under power.
Thanks. More thought about it I don't think its necessarily engine friction but more compression ratio thats the biggest factor. RustyK touched on it but it did not click.
When I pour a little fuel in the cylinder of a small gas engine then crank it w/o any load it will run away to some rpm then sputter out. It won't achieve the same rpm w/ load. And RPM would vary considerably to load. There is very little compression ratio in a small engine. However, I can gear down to 3rd and check my speed even towing going down a substantial hill with 21:1 compression ratio. So a diesel is not so sensitive to rpm runaway because it takes a lot of energy for the compression stroke.
It still doesn't quite make sense that it will blindly inject a set volume of fuel per IP stroke when there is no load requirement and RPM climbs. Maybe thats why the newer engines with slightly lower compression cut injections off with better feedback loop control. It gives better control and fuel economy too.
Some of my observations....
If I get out of throttle going down a big hill engine temp goes way down. Not sure what would happen if I could pick a lower gear and hold a steady higher TPS. I think temp will still cool off because with out any load its just going to pump air in and out and the fuel is just going along for the ride. As it takes a pressure resistance to build heat in the cylinder somewhat.
My shocks are nearly all worn out and springs, shackles, and bushings are getting softer with age. I can get the occasional easy spring wrap hop and my TPS is steady. Its not real severe and I have gotten better with recognizing it and getting out of it. When things were tighter I would agree it does get agrivated by TPS variance with foot being influenced by the truck hop. I think a mild case of spring wrap hop is the compression ratio thing mentioned above. Fuel volume probably never changes the variance of load causes the rpm variance. I have always wondered about the harshness of speed changes with the engine pushing or being pushed. And how some people the first time they drive my truck feel so rough. Dont ever click the cuise control off on a manual 6.5 running 70 mph plus its a heck of a jolt.
What you seem to be describing is 'way passed time for new Inj Pump and injectors' - 6.5 operation is smooth on\off pedal and will hold any desired rpm, once you get past the somewhat sensitive pedal (going from mech inj to EFI in the same truck, I had to run on cruise for a long while as I became acclimated to engine reaction to the pedal) - if rpm is erratic or varies for any steady pedal input, the DS4 is failing, which also means the injectors are beyond service life - something you should definitely check into
I don't necessarily think anything is drastically wrong seems normal to me. All I am describing is very subtle except taking off cruise. No bets on IP though. Injectors are only at most 40K miles. Timing chain is due. I was more or less interested in how it actually works control wise.
The spring wrap hop is rare and only happened a few times. I do get a little shudder on launch sometimes in 2nd, and some fishbite so I do think precise fuel metering is at question. But its not bad drivibility and maybe the noise suppression harness in wiring harness? I don't have any problems with cruise control its just harsh if you click it off on a manual truck. Its been like that since I have had it. I thinks its cutting fuel rate off immediately from a wind load to idle fuel volume and just backlashes through the drivetrain because the engine goes from pushing the truck to compression braking in an instance I figure an auto would unlock the torque converter and freewheel a bit and doesn't experience the same thing. If I match cruise control speed with TPS click off cruise and back out of throttle like normal driving its smooth and doesn't clunk/jolt harshly.
DmaxMaverick
02-19-2008, 11:07
The compression ratio is not a factor when compression braking. True, it takes more energy to overcome a higher ratio, but that energy gets put back on the power (combustion) stroke (the compressed gas pushes the piston back down, at the same rate it took to get it up there, with energy loss compensated for friction). Higher ratios just make for a more profound event. Remember, friction is more than just the mechanical meeting of parts. It includes the friction of the air getting compressed, and moving through the engine. When I refer to engine friction, it includes any collateral static load on the engine, and anything connected to it that takes energy to move ( or rotate). The gas engine with gas poured in the intake will run until the fuel is spent. A Diesel will do the same thing, if you pour fuel in the intake. Your pump/injectors should not be "pouring" fuel in the intake. Each injection event will be spent before the next event begins, so no runaway. If you are having a runaway condition, something is broke, malfunctioning or worn out. You CANNOT compare the characteristics of a gas engine to a Diesel engine, when we discuss fueling. The fuel is introduced to the combustion chamber differently, and it is combusted differently.
Regarding your observations.........
If you are on compression going down a hill (no throttle), the only heat generated will be from friction. Again, the air moving through the engine is friction. It generates heat. Your temp goes down because the cooling system is removing the heat at a much higher rate than it is generated. If you add a little throttle, there will be more heat. You probably won't notice it on a gage, because the fuel energy heat is still well below the capacity of the cooling system. If you throttle enough to generate enough heat to be realized on the gage, you will accelerate to max governed speed, and it will defuel.
One of 3 things will happen when you descend a grade:
1. Gravity (energy) will overcome friction, and you accelerate.
2. Friction will overcome gravity, and you decelerate.
3. Gravity and friction balance, and you maintain speed.
Inertia is also energy. When slowing your vehicle, it takes energy to overcome this energy. Either through engine friction (compression braking), or your service brakes (also friction). Service brake friction generates heat, also, but it is not absorbed by the engine cooling system. It is a factor with engine cooling, because using the service brakes relieves the engine of absorbing the heat of deceleration (less air friction from compression braking). Any time your vehicle is moving, it takes energy to overcome the inertia. Heat energy from compression braking, heat energy from service brakes, or gravity energy from ascending a grade.
Still, it's all about energy vs. friction, and vice versa. Friction is energy. Energy is friction. It is the trading of them that makes us go, and stop.
DmaxMaverick
02-19-2008, 11:25
The fishbite and/or shudder you experience on acceleration could be from several sources. Clutch chatter, spring/drivetrain wrap, inconsistent pedal pressure, valvetrain slack, etc, etc, etc. Too many possible factors to list. Adjusting your driving habit can minimize/eliminate it. You will either be covering up a malfunction by working around it, or cooperating with the nature of the system. Some people can't effectively operate a manual tranny vehicle, while it's easy for others.
What you experience with cruise control engaged is completely different than manual throttle control. On cruise, I think you will find some throttle input happening (PCM) that causes your inconsistent smoothness. Could be normal, or the PCM throttle control compensating for a deficiency. You'll have to connect a scanner and monitor your throttle input to see it. There is no functional difference when on cruise, beyond the PCM controlling the throttle. Other systems do not act any different because cruise is enabled. In the same way your PCM controls throttle when on cruise, another driver will operate your vehicle different than you. Sit in the passenger seat and have someone else drive. It will be a different experience, and a different end result.
I'll have to chew on the compression ratio not making a significant difference on compression braking. It seems to me it does. I think its much harder to "push" a higher compression engine. Might be more energy put into compressing the air charge and heat dissipation that might be an energy conversion that some is lost if combustion energy is really low. And or could be considered friction don't know.
Just to clarify its not the cruise control operation thats the problem. It works fine smooth holds speed etc. Its disengaging it thats the problem. If I slide my foot off the left side of the throttle (like I am dumping a clutch) from say 30% against a wind speed or load it jolts and its a harsh transition of speed too. Which I think comes from the compression ratio not wanting to be "pushed".
93GMCSierra
02-19-2008, 13:17
I have the 5 speed transmission as well, and a mechanical injection pump and yes any time you have a sudden decrease in accelerator pedal you will get that harsh slow down, I agree with an auto that is cushioned alot more, but I think even on gas engines that is the case.
The 'slam' you describe when disengaging the cruise (either by brake or switch) seems to be inherent to the manual trans. 6.5s. Mine has done this since new. I get around it by slightly depressing the accel. before disengaging the cruise. Much smoother.
Seems GM's programming is too much of an on/off switch in this respect...
Joe.
DmaxMaverick
02-19-2008, 16:43
The 'slam' you describe when disengaging the cruise (either by brake or switch) seems to be inherent to the manual trans. 6.5s. Mine has done this since new. I get around it by slightly depressing the accel. before disengaging the cruise. Much smoother.
Seems GM's programming is too much of an on/off switch in this respect...
Joe.
That's a key, and not just with GM Diesel engines. When you go from on the throttle to off, the fuel will stop until it approaches the minimum governed speed. It is more abrupt with a Diesel for this very reason. On a gasser, the fuel/air mixture is a greater volume, because the cylinders must use up what's in the intake and runners before it is "defueled". Like said before, the Diesel gets one fuel event per combustion event, at the time of that event. Stopping the fuel stops it immediately. Only the rotational inertia of the engine slows down the deceleration process. You can see this with a scanner. Watch the fuel delivery volume when you go from WOT to off pedal. It will go to zero until it approaches idle speed. If it didn't, your engine would never slow down to idle speed, unless the load were enough to slow it. Some fuel injection designs have a wider envelope around governed speeds, allowing a more smooth transition, but it is still essentially the same.
The compression ratio is irrelevant to compression braking because that greater compression is returned on the down side of the stroke. All that compressed air will then push the piston down. It is harder to turn a high compression engine because you are feeling mostly only the compression stroke. A Diesel engine takes more energy (be it an electric starter or your muscles) to rotate because of its rotational mass and larger bearing surfaces, compared to a gasser. All that translates into friction, which is where we started the discussion.
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