Gas Exhaust in 4 Stroke Engines: Continuous or Discrete?

[SmritiB]

Is the gas coming out of the exhaust during the working of a 4 stroke engine continuous as long as the automobile moves? I know the intensity of air used depends on the speed, but the air that exhausts out, does it come continuously in time or like a discrete function ?

 

[deskswirl]

It comes discontinuously in time determined by the opening of the exhaust valve and expulsion of the fuel cycle reaction products. But of course when the engine speed is increased (as it is when acceleration occurs) then you may consider the exhaust to be a continuous, constant pressure flow.

 

[Ketch22]

It is a pulsating flow. With each exhaust valve event there is a pressure wave and a peak flow. As it travels down the exhaust pipe this becomes normalized. With a multi cylinder motor the pulses are all at different timings. With two crankshaft revolutions there are pulses equal to the number of cylinders. As this flow travels down the exhaust pipe and through the muffler and other items the pulses diffuse and at arrives at the tail end in a relatively constant flow. Given a modern well muffled consumer vehicle, modifications also change this flow pattern.

 

[SmritiB]

okay,
got it,
Thankyou,

 

[SmritiB]

I would also be glad if anyone could tell me if this working cycle which starts with intake of gasoline and is terminated by the exhaust valve's opening has any direct connections with the rotations of the wheel ?

 

[deskswirl]

It is direct. However, establishing a one-to-one correspondence between an engine rotation and a rotation of the wheel is difficult without specific knowledge of gear ratios of both the transmission, rear end/transaxle and final drives.

As an example many passenger cars have a gear in the transmission which which allows for overdriven. This means that for one rotation of the engine more than one rotation of the wheel is made. Typically this gear is shifted into (manually or automatically) when the cars speed meets a minimum velocity. It provides a way to increase the speed of the automobile without increasing the engine's angular frequency without bound (leading to poor combustion and ultimately engine failure). To calculate the necessary size of this gear multiple other measurements must be known (flywheel size, transmission output shaft gear size, pinion and ring gear size in the rear end, wheel and tire size, etc.).

Big picture is that the connection between a single combustion cycle and rotation of the wheel is highly specific on the design. Which is why most internal combustion engines are rated/tested/evaluated with measurements taken at the flywheel. In common usage is brake mean effective pressure (BMEP) and torque. Given the engine's displacement, BMEP and with knowledge of the amount of fuel given over a period of time one can define things like efficiency.

If you'd like to know more go to your university library and look for Engineering Fundamentals of the Internal Combustion Engine by Pulkrabek or Internal Combustion Engine Fundamentals by Heywood. Both (especially the later) provide extensive description as to how chemical energy can be harnessed in prime movers.

 

[SmritiB]

Okay,
That was indeed helpful.
Thank you so much.