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Internal combustion dynamics

  1. Jan 25, 2015 #1

    I am interested to know in detail, how internal combustion engines work dynamically, ie. as the demand for speed and torque change

    what happens exactly? I know that when we want more speed/torque, we push the gas pedal, but what happens exactly between that moment and the increased rpm of the engine?

    different amount of fuel in the chamber, results in faster rotation of the piston? can I see a graph of this relation?
  2. jcsd
  3. Jan 25, 2015 #2


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    Basically, the engine is throttled after it has started by restricting the amount of air which can flow into the cylinders. There is a plate fitted into the air intake which can rotate, such that either the opening is mostly closed, in which case the engine is running at very low speed, or the opening is such that the airflow almost entirely unrestricted. The gas pedal is directly connected to this plate, and the operator of the engine opens the throttle plate wider by pressing on the pedal with his foot.

    An engine is designed to run with a set amount of fuel in proportion to the amount of air entering the engine. More air = more fuel = faster engine RPM = more power.
  4. Jan 25, 2015 #3
    is there a mechanical mechanism to trigger the ignition according to different rpms? what is the mechanism exactly?

    also, can I know the graph of the air/fuel intake and speed/power of piston?
  5. Jan 25, 2015 #4


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    With a purely mechanical ignition system, a distributor was driven by the engine, and the distributor had a special cam fitted which was timed to open a switch when a certain cylinder had a compressed mixture of fuel and air inside. When the switch opened, it would allow a high voltage current created by an electrical induction coil to flow into the spark plug wire, into the spark plug in the cylinder. The current would jump the spark gap and ignite the fuel mixture.

    With an electronic system, mechanical switches are no longer used to trigger the current to each cylinder. Instead, a magnetic pick-up senses the position of the crankshaft of the engine and fires the spark plug in the proper cylinder.

    It's not clear what you mean here.

    Instead of playing Twenty Questions about internal combustion engines, do some research first. Remember, we have been discussing Spark Ignition, or gasoline, engines so far. Compression Ignition, or diesel, engines are internal combustion engines, too, and they function slightly differently.

    Here are some articles to get you started:





    Whole textbooks have been written on the design and performance of automotive internal combustion engines. Good Luck! :)
  6. Jan 25, 2015 #5

    jack action

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    First, let's start by stating that all you need to control is the torque of the engine. If you increase the torque to a point where it is greater than the load on the crankshaft, it will result into an acceleration of the crankshaft rpm (and vice-versa).

    To control the torque of the engine, you need to vary the force applied on the piston. You do so by varying the amount of fuel burned in the combustion chamber.

    There are basically 3 ways of accomplishing this in typical internal combustion engines:

    You vary the amount of fuel injected in the cylinder. This is how a diesel engine works. When you press the gas pedal, more or less fuel is injected into a cylinder filled with fresh air. The difficulty with that method is to properly mix the fuel with the air to obtain the proper air-fuel ratio for a complete combustion; thus only part of the air is used for the combustion and that is why, for a given power output, diesel engines have generally a larger displacement compared to petrol engines.

    You vary the amount of air going into the cylinder. This is how a petrol engine works. When you press the gas pedal, more or less air is permitted to enter the cylinder. The fuel is pre-mixed with the air - in the proper ratio - prior to enter the cylinder (or while it enters the cylinder).

    You vary the amount of power strokes per cycle.
    This is a method that was used on early industrial petrol engines. It is referred to as the hit-and-miss cycle. There is no gas pedal per say. The engine is auto regulating itself to maintain a certain rpm. When the engine tends to go below that rpm (either because the engine load was increased or the friction alone slows down the engine), the fuel mixture is permitted to enter the cylinder and completely fill it. This gives a huge explosion that accelerate the engine's rpm. And after a few revolutions of «freely» rotating, the engine slows down again and the cycle repeat itself. Here's a video of such an engine, first with a full [electrical] load, then the load is reduced (unplugging the electrical load) and we can see (and hear :)) that the number of strokes per cycle needed to keep the same rpm decreases:

  7. Jan 26, 2015 #6
    thanks for the great info

    how much energy is stored in a ml of gasoline (between the molecules) ?
    how much energy we get out of combusting that ml?
    what is the maximum percentage of combustion we can achieve? I suspect we cannot have 100% total combustion of a ml of gasoline
  8. Jan 26, 2015 #7

    jack action

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    From the Automotive Handbook (4th ed. p. 232):
    As you can see this is the figure for the amount of energy per volume of air going into the cylinder and not per volume of fuel like you ask for. The later can vary from fuel to fuel, but when you consider the air-fuel ratio needed, it all comes down to basically the same value.
    I think you mean «what is the maximum efficiency we can achieve? I suspect we cannot have 100% efficiency» and I would refer you to this Wikipedia article. Basically, an engine working with the Otto cycle (petrol engine) can theoretically convert 56-61% of the heat energy into mechanical energy. But since there are many departures from ideal behavior that waste energy, actual values are closer to 35%. You might expect about an extra 5% from an engine using the Diesel cycle.
  9. Jan 26, 2015 #8
    thanks but I don't mean exactly that

    I mean this:
    I have read that in the engine, there is not always 100% combustion of the 14.5:1 stoichiometric air/fuel mix
    some of the hydrocarbons are not burnt or not fully burnt

    what are the current percentages of combustion that we achieve and how it can be increased?
  10. Jan 26, 2015 #9

    jack action

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    Some info gathered from Design and simulation of two-stroke engines by Gordon P. Blair (1996, p.303-305):

    The combustion efficiency (ηc) can be defined as:

    ηc = ηo ηλ ηSE

    Where ηo is an overall efficiency that express the incomplete combustion due to incomplete flame travel into the corners of particular combustion chambers, weak or ineffective ignition systems, poor burning in crevices and flame decay by quenching in most circumstances. Its value is between 0.85 and 0.90.

    The scavening efficiency (SE) is defined as the mass of fresh air with respect to the total mass trapped inside the combustion chamber prior to the combustion; since exhaust gas may not have completely escape the cylinder during the previous cycle. The value of ηSE varies between 0.73 and 1.00. With well-designed 4-stroke engines, you can assume ηSE = 1.00.

    The air factor (λ) represents the quantity of air versus the quantity of fuel present in the air-fuel mixture. The air factor affects combustion in 2 ways.

    First, the more fuel is present, the more chance there will be that every oxygen molecule is involved in the combustion process. Although, too much fuel will alter proper fuel distribution in the mixture, thus dropping combustion efficiency.

    Second, even if combustion efficiency is dropping while reducing the air factor, the air-fuel ratio (AFR) will also drop, but at a slower rate. Hence,the heat available will increase (until a point), even if combustion efficiency is not at its maximum value.

    The combustion efficiency due to the air factor (ηλ) is different for SI (petrol) and CI (diesel) engines:

    SI engines achieve a ηλ = 1 when λ is about 1.12. CI engines achieve a ηλ = 1 when λ > 2.00.

    SI engines achieve a ηλ = 0.87 when λ is about 0.875 (when maximum heat is released). CI engines achieve a ηλ = 0.83 when λ = 1.25 (when maximum heat is released). Although, in the case of CI engines, there will be high level of black smoke at this value (see image below); Typical street vehicles won't probably go less than 1.65 for maximum performance (ηλ = 0.95).

  11. Jan 27, 2015 #10

    so failing to achieve a complete combustion is not the bottleneck of energy loss in IC engines
  12. Jan 27, 2015 #11
    More than incomplete combustion, the major loss is the heat loss. Thermal efficiency of IC engines is about 35%. Energy is lost due to conduction (heating up of piston and engine cylinder) and radiation.
  13. Jan 30, 2015 #12
    when we are NOT pressing the gas pedal, there is still flow of air/gas and combustion inside the engine?
    just minimum to rotate minimally the engine?
  14. Jan 30, 2015 #13

    jack action

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  15. Feb 2, 2015 #14
    why do we need an electric motor to start the petrol motor?
  16. Feb 2, 2015 #15
    How will you give the piston the first push before the fuel starts burning?
  17. Feb 2, 2015 #16
    mmm I see...
  18. Feb 2, 2015 #17

    jack action

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    Except when engines become too big, you don't really need a starter. A lawnmower or a small motorcycle is started with a pull cord or with a kick start.

    This shows how easy it can be to start an engine with a kick start:

    This is why we invented the starter:

  19. Feb 2, 2015 #18
    yeah, but I am refering to cars mainly, car engines are considered larger, right?
  20. Feb 2, 2015 #19

    jack action

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