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Mechanism of Intake Pumping Losses

  1. Jun 29, 2014 #1
    I'm a mechanic and I'm debating the exact mechanism of the so called "pumping losses" or "pumping work" of conventionally throttled 4 stroke spark ignition engines.

    We know that EGR is employed to reduce intake pumping losses and actually increases engine efficiency for slightly better fuel economy, but we disagree about how.

    My claim is that higher intake manifold pressures actually transfer energy to the piston on the intake stroke.

    My feeling is that if there is an extremely high throttle restriction, and lets say, 30"hg of intake manifold pressure, the engine will lose more energy on the intake stroke as it *drags* the pistons down the cylinders.

    I'm basing it on the surface area of the pistons, and the engine speed. I feel the inefficiency created by throttling comes from discarding the energy in the atmospheric air.

    Can't have something for nothing. My colleagues seem to think the energy is lost across the throttle.

    I can understand a reduction in volumetric efficiency due to the throttle, but not combustion efficiency.

    What is the truth, really? Where is the energy wasted on an engine without EGR? Is it related directly to MAP, or to the throttle angle?

    Dying to know.

    Thanks,
     
  2. jcsd
  3. Jun 29, 2014 #2

    SteamKing

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    All real fluids (like air, water, etc.) take energy to move through a pipe or other channel. The engine in an automobile can only move so much air into the cylinders to be burned every combustion stroke. If engine A has a less restrictive intake than engine B, engine A is able to burn more fuel and make more power than engine B.

    EGR stands for 'Exhaust Gas Recirculation'. It is a pollution control device fitted onto gasoline engines and some diesels to return a small amount of exhaust gas back into the intake stream. It's not there to reduce intake pumping losses; it's supposed to reduce to formation of oxides of nitrogen in the exhaust of the engine:

    http://en.wikipedia.org/wiki/Exhaust_gas_recirculation

    The energy due to a higher intake manifold pressure is small compared to having more air in the cylinder with which to burn more fuel.

    Regular atmospheric pressure is 29.92" Hg. A throttle restriction of 30" Hg would suggest that the throttle is closed tightly, and no air could enter the engine.

    There is some energy lost due to throttle restriction; this is accepted because the throttle is used to control the speed of the engine.

    If the engine suffers reduced volumetric efficiency, it means that not enough air can get into the cylinders to burn the fuel. Power output and the overall efficiency of the engine drop as a result.

    Turbocharging or supercharging an engine is a means to cram more air into the cylinders than would otherwise be drawn in naturally.

    More air = more fuel = more power

    which is why a small turbo motor can put out more power than a large non-turbo motor.

    The throttle is put into the intake stream as a means for the operator of a vehicle to control how much power the engine is delivering in order to regulate the speed of the vehicle. The MAP ('Manifold Air Pressure') is a sensor which tells the engine control unit how much fuel to meter to the engine for complete combustion.

    Believe it or not, cars used to work fine without EGR and all the other pollution controls. Most of these devices have been mandatory on cars since the 1970's to reduce emissions.

    Only about 25% to 35% of the energy in the fuel is converted to useful work in a gasoline engine; a slightly greater proportion of work is obtained from a diesel engine. Some of the energy is lost due to friction occurring inside the engine. Although lubrication reduces friction, it does not eliminate it. Most of the energy which is lost goes out the tailpipe or into the cooling system.
     
  4. Jul 5, 2014 #3
    Well, I think more research needs to be done here.

    My view, we have exhaust, head ports, piston crown, cylinder head layout, cam and valvetrain, intake route and filtering quality.

    Those off the top of my head will be the 'mechanism' IMO. Each fresh charge filling the cylinder will be dictated by the exhaust system's efficiency on evacuation as well as the camshaft's timing to scavenge the cylinder upon the exhaust stroke as well as the valve & port flow quality not just at low lift but high lift increments too. There are many points that need to be seen, and, treat the system from tailpipe to throttle body.
     
  5. Jul 5, 2014 #4

    jack action

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    As said earlier, this is wrong. EGR is used to replaced air-fuel mixture with already burnt gas such that the energy released by the combustion will be smaller, leading to a smaller combustion temperature. High combustion temperature creates NOx which is a harmful pollutant.

    What really happens is that if the throttle is closed and no air can enter in the intake and combustion chambers, as the piston goes down, the pressure will decrease (vacuum). But because the pressure in the crankcase (on the other side of the piston) stays the same (i.e. the atmospheric pressure) this creates a net force «pulling» the piston upward, against its downward motion.

    When there is no restriction, the pressure inside the cylinder always stays the same (i.e. the atmospheric pressure) since there is always «new» air getting in to fill the «new» increased cylinder volume.

    But, as a good news, if the valves are closed when the piston is at BDC, this lower pressure inside the cylinder would still help «pulling» the piston up; thus cancelling the previous loss during the downward motion. But the intake valve is rarely closed when the piston is at BDC, so the cylinder keeps getting filled as the piston goes up and pressure keeps increasing as a consequence.

    This would correspond to the «drag» you are talking about.

    This is also true. The throttling due to the flow resistance in supply lines, heat exchangers, regenerators, and other components of (thermal) machines is a source of losses that limits the performance.

    When the air is throttled, you might have noticed that the throttle valve gets cold. This difference in temperature leads to some losses for the fluid.

    They are also some losses due to turbulence around the butterfly valve.
     
  6. Jul 7, 2014 #5
    EGR definitely does contribute to fuel efficiency on a gasoline engine and even on a diesel engine.

    It is fairly obvious that on a gasoline engine, there are pumping losses at the throttle due to the air being forced through a turbulent restriction. The engine is more efficient at higher throttle openings as long as other factors like A/F ratio and rpm don't get out of the efficient range. This is one of the main reasons a lower power engine is more efficient working harder than a more powerful engine hardly working.

    EGR allows the throttle to be opened more to allow the same amount of Oxygen in for combustion, since part of the air is being displaced by air with little to no oxygen and the manifold vacuum is decreased greatly, decreasing the pressure drop across the throttle which decreases airflow at that throttle position, requiring more throttle to be added for the same cruise speed or slight acceleration. There are less pumping losses at a higher throttle opening than a lower one, and the reduced pressure drop even at the greater opening angle reduces velocities and turbulence .

    A lot of modern fly by wire engines, with or without Direct Injection will run at WOT(full throttle) at low loads, even normal cruise speeds down to 30-40mph. This is made possible by EGR, valve timing and lift control, leaner A/F ratios and in the case of Direct injection: fuel timing(injection start time).

    If you take an engine at a constant speed and open the EGR what happens? the RPM drops, right?
    If the pressure in the manifold was adding energy to the piston on the intake stroke, the rpm would go up, right?
    no it goes down, so to maintain the same speed, you must increase the throttle, which reduces the pumping losses.

    EGR can also increase efficiency by tranferring some heat to the charge, which may evaporate more fuel to make combustion more efficient.

    Now, someone else tell us how it helps efficiency on a diesel.
     
    Last edited: Jul 8, 2014
  7. Jul 8, 2014 #6
    After rereading your post, I think what you are describing about the increased manifold pressure(decreased vacuum) reducing the drag of the pistons on the intake stroke would be true under deceleration, as the throttle is completely closed, and the main engine braking action is due to this drag. any reduction in vacuum will decrease this, but there is no fuel being consumed at this time, unless you have a carburetor. Engine braking can hurt fuel economy though, just like using the regular brakes. Any energy wasted slowing the car down is energy that could have moved the car further without using any more fuel. Most automatic transmission cars have pretty much eliminated engine braking for this reason.

    When the engine is producing power, as in a steady state maintaining speed or accelerating, the reduced vacuum reduces airflow into the engine, requiring the throttle to be opened more to maintain speed, which of course reduces vacuum even more. the reduced pressure drop and more open throttle reduce the pumping losses considerably.

    I have an old TBI chevy truck, and my EGR went out(electronic valve). The fuel mileage did suffer.
    I went from averaging 16-18mpg on road trips to 13-15mpg. It was remedied with a new valve, and has returned to 16-17mpg. I haven't hit 18 again yet, but I think it is just a matter of other conditions like temperature, wind, etc.
    This example is for an old crude Throttle body injection system. It does not take advantage of the egr as much as a fly by wire system that is designed to run a stratified charge. The newer designs utilize the egr, along with a leaner mixture, and other factors to allow the throttle to be left wide open and a surprising range of power output can be had at that WOT by varying the fuel and egr, cam timing, etc. n The TBI engine may benefit more from the heat transfer to the air/fuel mixture though, since the fuel is already in the air when it gets there.

    If you get the opportunity, take an OBD scan tool, and hook it up to a modern stratified charge car's obd2 port and monitor throttle position while cruising down the road. You'll see the ecu put that throttle wide open as soon and as often as it can.

    Note: for your safety and others, get someone else to drive while you watch the scan tool.
     
    Last edited: Jul 8, 2014
  8. Jul 11, 2014 #7
    one way how EGR can help fuel consumption is when modern EGR are water cooled. EGR in new cars can recirculate quite large amounts of exhaust fumes and they are cooled by water. so it helps to warm up engine and thus decrease fuel consumption.
    and some people may think this is not big deal, but for small (less then 2l) diesel engines it helps hugely. small diesel has so little waste heat, that in winter there can be problem to warm the engine to operating temperature at all.
     
  9. Jul 11, 2014 #8
  10. Nov 24, 2015 #9
    Throttle position pumping losses is a load of heuy. With the throttle closed and the engine spun, it is doing no work. Open the throttle and now the engine is pumping air. Watch how the vacuum cleaner motor speeds up when you cover the suction pipe. It speeds up because it is doing less work.
     
  11. Nov 24, 2015 #10

    JBA

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    You obviously do not know anything about the performance difference between a positive displacement compressor and a centrifugal blower.
     
  12. Nov 24, 2015 #11

    Randy Beikmann

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    autodoctor911 has the right idea. I'll try to summarize my own thoughts, which are very similar (and will repeat some of his points). I'll start with a 4-stroke internal combustion engine with the valves timed to open or close at TDC or BDC. The crankcase is always at atmospheric pressure (it doesn't actually matter, but makes it easier to discuss). We'll also assume the exhaust is at atmospheric (no back-pressure). We'll ignore valve restriction and friction.

    First let's try wide-open throttle, beginning with the intake stroke. With no throttle restriction, the pressure in the intake manifold and the cylinder are at atmospheric (same as the crankcase), so there's no pressure difference across the top/bottom of the piston. This means there is no force on the piston during the intake stroke, and it takes no work to bring the air in. You then compress the air/fuel mixture (which requires some work), burn it "instantly" at TDC, then let it expand while pushing down on the piston. This produces work (the whole goal of this!). We open the exhaust valve at BDC and let the pressure blown down "instantly." Then during the exhaust stroke the cylinder pressure is atmospheric, so the pressure difference across the piston is zero, the force is zero, and it takes no work to let out the exhaust. * WOT is very efficient because it takes no work to "pump" the air/fuel mixture in. *

    But WOT usually makes too much torque for cruising. If we reduce the torque by throttling the air inlet (and reducing the heat of combustion), the manifold and cylinder pressure are less than atmospheric - and less than the crankcase. Now the pressure difference across the piston is pushing it upward in the cylinder, while the piston is traveling down. The crankshaft now has to do work to pull the piston down - this is the pumping loss. Torque is now actually reduced for two reasons: 1) less work done by the smaller amount of air/fuel mixture, and 2) the pumping loss. It's breathing through a straw.

    Now let's reduce the torque by keeping the throttle wide open, but mixing enough exhaust gas into the air/fuel mixture to get the torque we want. Now the cylinder is at atmospheric pressure, and there is no pumping loss. The engine is as efficient now as it is for WOT.

    The main point: When you need reduced torque, reduce it as much as possible by other ways, before reducing the throttle opening:
    - EGR
    - Close the intake valve late, so that some of the air is "spit back" into the intake manifold.
    - Increase the intake/exhaust overlap duration, so more exhaust gas stays in the cylinder for next time.

    There are other ways, but you get the idea.
     
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