Episode 18: How Motors Work


Episode 18: How Motors Work

This show is all about motors. 4-Stroke & 2-Stroke motors. The kind we all use on our motorcycles.  In this show we describe the important components in a motor (we’ve done shows detailing most of these components you can refer to), and how they interact to complete the strokes that keep a motor turning.  Make sure to follow along with our outline below, and look at the reference videos, pictures and links to really get a good understanding of what we’re talking about. It’s very hard to understand this topic without seeing a video depicting how the components interact.


Jacob O’Neal’s ‘How a Car Engine Works’ animations:
This website has some amazingly designed animated GIF’s showing how motors work. While they’re specifically about car motors, this definitely helps illustrate what we’re talking about. I’m posting the images below since they are freely available online. All credit goes to Jacob O’Neal. Click on the images below to view them full size.









How 4-Stroke engines work by Mercury Marine:


How 2-Stroke engines work by How Stuff Works:

Show Notes & Outline

An engine needs 3 things to run:
1) Mixture
2) Compression
3) Spark

4 Stroke Motors:
1) SUCK – Intake
2) SQUEEZE – Compression
3) BURN – Power
4) BLOW – Exhaust

Components in a 4-stroke motor:

  • Carburetor(s)
  • Head
  • Valves
  • Cam(s)
  • Combustion chamber
  • Ignition system
  • Piston(s)
  • Connecting rod(s)
  • Crankshaft
  • Exhaust

How it works:

Intake Stroke:
The intake valve opens as the piston goes down
Creating a vacuum, draws mixture from the carburetor thru the head and into the cylinder
The intake valve closes as the piston reaches the bottom of the stroke

Compression Stroke:
Both the intake and exhaust valves are closed
The piston travels upward compressing the mixture to the combustion chamber

Power Stroke:
Valves remain closed
A spark from the spark plug ignites the compressed mixture
The burning gases expand, pushing the piston down with force

Exhaust Stroke:
The exhaust valve opens as the piston goes up
Burnt gases are blown out of the cylinder thru the head and out the exhaust pipe
The exhaust valve closes as the piston reaches the top of the stroke

Then the process starts over again.

2 Stroke Motors:

No valves
No heads
Fires once per revolution
Few moving parts
No oil in the crankcase
Piston acts as valves

Components in a 2-Stroke Motor:
Intake port
Reed valve
Ignition system
Combustion chamber
Exhaust port
Connecting rod

Mixture – Air/Gas/Oil:
Oil in the mixture lubricates and cools moving parts
Premix or oil injection from separate oil tank
Mixture from carburetor fills the case

How it works:
With mixture in the case and the piston at the top of the stroke after a spark
Piston travels down exposing exhaust port releasing most of the burnt gases
As the piston travels farther down it pressurizes the case and exposes a path from the case allowing mixture to enter the cylinder

As the piston travels up it forces more burnt gases out and closes the passage to the case
The vacuum the piston creates opens the reed valve drawing more mixture into the case
The piston moves up more blocking the exhaust and compresses the mixture
When the piston reaches the top of the stroke the spark plug fires and ignites the compressed mixture
Forcing the piston back down closing the reed valve and starting the process over again

One comment
  1. Jeff

    Good episode guys. I especially enjoyed the 2-stroke explanation & discussion – never really looked into this engine before.

    In regards to exhaust- I never understood how going from a 4 into 4 to a 4 into 1 exhaust would improve engine performance either- that is until I read the Cafe Racer Guide by Jim March you posted. “Here’s why there’s a performance boost:
    Take the case of a twin (two-cylinder). Set up right, when the exhaust pulse from one cylinder hits the
    merge point, the pulse from the other cylinder is just starting. Once past the merge point the first
    cylinder’s pulse literally pulls the pulse from the other cylinder. And when that one clears the merge,
    the cycle starts again. Same with triples and four-bangers.” I thought that was pretty cool.

Leave a Reply

Your email address will not be published. Required fields are marked *