[xBhp Universal Thread]: How an Engine works ?

[Abhisek]

Guys, though I am a student from commerce stream even though I am always interested about - How engines work and what components are required by an engine to perform ?
The utmost basic name of the components like the piston, cam - shaft, gears, transmission etc. are almost known to most of us. So why don't we discuss in detail to every aspect of the engineering involved in making of a engine. After all there is no end to the curiosity of a human being, so can't we start a discussion with a very simple 4 stroke single-cylinder engine of say 100-150 cc & enhance our knowledge regarding a very important part of our life which we all admire.

[Samarth 619]

Topic Approved.
Hi Abhishek,

Most members are aware of how an engine works, and I'm myself from commerce stream.

In a 4 stroke engine: A carburetor mixes pure air with petrol in the right ratio, and provides this mixture to the cylinder. 1 valve or 2 valves let this mixture enter the cylinder, and then this valve closes.



A spark plug emits a spark and burns this Air fuel mixture. This expansion causes the piston to be pushed down. This piston when pushed down, rotates a crankshaft attached below the piston. And then another valve or set of 2 valves emit the burnt gas, and direct it towards the silencer pipe.
Then this process is repeated and it happens in the blink of an eye.

This piston is attached to a crankshaft, which rotates due to up-down piston movement. And this crankshaft further transfer the power through clutch assembly, gearbox and finally to the rear wheel through a chain.

Now, this is just a basic explanation and you'll have to learn a lot more using Xbhp forums. All the best.


1. Piston is a regularly moving component, once in a direction and the next moment in the inverse direction. So it must be light and strong too.

2. Camshaft: It decides opening and closing of valves. It can operate both inlet and outlet valves (Single overhead camshaft: SOHC) or either one of them (Dual Overhead camshaft: DOHC) depending on its design. DOHC is often better in top end performance.

3. Gears: Ratios. Providing the balance between acceleration and speed/ rpm. Gears decide the relation between crankshaft's speed and rear wheel's speed.

[Mad Mik]

In a 4 stroke engine: A carburetor mixes pure air with petrol in the right ratio, and provides this mixture to the cylinder. 1 valve or 2 valves let this mixture enter the cylinder, and then this valve closes.

The wording on that pic is very confusing. That pic looks like a 2 stroke with the air intake on the side, not through the valve in the top of the combustion chamber like a 4T.
It needs rewording, also the fuel injector looks to be in the place of a sparkplug.
A lot of EFI models have the injector squirting directly onto the back of the intake valve or into a tuned length runner (tuned from the factory) unless it is a specific direct injection (similar to diesel)

Work cycle of 4-stroke internal combustion engine 3D - YouTube

Four Stroke Engine Basics
Take a look at this link and use the slider on the right to slow it down to the slowest setting.
Note this is not a overhead valve (OHV) type head, but the principal remains the same. Watch how the valves open and close in relation to the pistons stroke. Look at how the cam revolves. RPM is a measurement of the cam's revolution, Not piston's stroke/crank revolutions.

Basically a car/bike 4T internal combustion engine is a airpump.
the carb is a fuel metering device.

Stroke 1 Intake
Air + fuel is sucked into the engine by the piston on its downward stroke with the intake valve/s open,

Stroke 2 Compression
Depending on the cam timing the intake valve/s will close and piston will begin to travel up squeezing and compressing the fuel air mix.

Stroke 3 Power
Depending on spark timing, there will be a pulse sent to the sparkplug XXdegrees before TDC (top dead center with regards to where the piston is on its upward stroke, with TDC being as it states, the most upper part of the pistons verticle journey) Resulting in a explosion of the fuel/air mixture.
The timing is crucial as you want the explosion to be at its peak when the piston is at TDC, the resulting force will push the piston down very strongly due to the sudden expansion of the air in the explosion,

Stroke 4 Exhaust
Piston is at the bottom of its stroke and begins to travel upwards.
Exhaust valve/s open allowing the spent combustion gasses out of the cylinder

Before the piston reaches TDC the intake valve/s may begin to open, and the whole 4 stroke cycle begins again.

You may have some "valve overlap" in your engine.
This term refers to both intake and exhaust valve/s being open at the same time.
When you see race cars shoot flames out their exhaust when downshifting and braking this is due to valve overlap. Since both the intake and exhaust valve/s are open, unburnt fuel/air mixture gets pushed out into the hot exhaust, resulting in a explosion and the flames you see are a result of that.

Installing a different profile cam can increase power and torque, However unless you have something like Hondas V-tech cam system (with multiple lobed operating at different rpm driven by water temp, oil pressure, rpm) then you have to decide where you want your power/tq increase. Be it low/mid/top end.

If you want to make more HP then you basically just need to make the engine be able to "suck" more air and expel more air out. Sounds easier than it looks.