Two and Four Stroke Engines

**bear** · 23rd November 2004, 15:05

So what is the difference between two and four stroke bikes. When I've been telling some old fellas that I got a new bike recently and they all ask if it is two or four stroke.

Were two strokes the only way to go at some stage, and fours a relatively new thing? I thought most road bike nowadays are four strokes.

**Hitcher** · 23rd November 2004, 15:06

Sigh. I must be getting old.

I shall wear the bottoms of my trousers rolled...

**Motoracer** · 23rd November 2004, 15:10

A 2-stroke engine doesn't have valves, a 4-stroke engine does.

In combustion engines the inner energy set free by combustion (e.g. of gasoline or Diesel fuel ) is changed partly into mechanical energy. There are several varieties of combustion engine. The most common is the four-stroke Otto Engine, which was invented by the German merchant and engineer Nikolaus August Otto in 1876.

The working mechanism of the four-stroke Otto-Cycle: The engine's cylinder has got two valves, the intake valve and the exhaust valve, which are opened and closed by a camshaft mechanism. A crank moves the piston in the cylinder bymeans of a "connecting rod".

The operation of the engine is divided into 4 parts, which are called strokes:
-1st stroke (Intake): The piston sucks in the fuel-air-mixture from the carburetor into the cylinder.
-2nd stroke (Compression): The piston compresses the mixture.
-3rd stroke (Combustion): The spark from the spark plug inflames the mixture. The following explosion presses the piston to the bottom, the gas is operating on the piston.
-4th stroke (Exhaust): The piston presses the exhaust out of the cylinder.

In a two-stroke engine (e.g. in lawn-mowers or in power saws), the engine is operating every second stroke - there are no valves. It is necessary to use a mix of gasoline and oil (two-stroke oil) as fuel. This is used to lubricate the piston and the crank shaft.

1st stroke
The compressed fuel-air mixture ignites and thereby the piston is pressed down. At the same time the intake port is covered by the piston. Now the new mixture in the crankcase becomes precompressed. Shortly before the piston approaches the lower dead centre, the exhaust port and the overflow conduit are uncovered. Being pressurized in the crankcase the mixture rushes into the cylinder displacing the consumed mixture (exhaust now).

2nd stroke
The piston is moving up. The overflow conduit and the exhaust port are covered, the mixture in the cylinder is compressed. At the same time new fuel-air mixture is sucked into the crankcase

Note: I didn't write the above. I copied and pasted it from a google search. You should try that sometime (searching for info on google that is).

**Coyote** · 23rd November 2004, 15:12

Easiest way to tell between the two in most cases is a 2 stroke has a whiney sound, and a 4 stroke has a deeper sound. Also, most 2 strokes have the huge pipe to the side of the engine.

2 strokes are damned good fun. My KX is so awesome.

**duckman** · 23rd November 2004, 15:13

Originally Posted by Hitcher

Sigh. I must be getting old.

I shall wear the bottoms of my trousers rolled...

YUP!! - Ya know that rolling your trousers up is very "in" at the moment ... wait wait... it just went "out" again ....

**bear** · 23rd November 2004, 15:15

Yeah, good point on the google searching, but were most road bikes two stroke at one stage and then went to predominately four stroke engines?
Was interested as to why the more mature folk were asking the 2/4 stroke question.

**bungbung** · 23rd November 2004, 15:25

Originally Posted by Hitcher

Sigh. I must be getting old.

I shall wear the bottoms of my trousers rolled...

So as to blend in with today's yoof, you will need to make certain one leg is rolled higher than the other depending on your eastside/westside allegiance

**James Deuce** · 23rd November 2004, 15:39

Originally Posted by bear

Yeah, good point on the google searching, but were most road bikes two stroke at one stage and then went to predominately four stroke engines?
Was interested as to why the more mature folk were asking the 2/4 stroke question.

Nope. A lot of cheap and small capacity bikes were two strokes, but mid-90s emissions regulations meant that they couldn't pass various air quality tests, despite such advances as direct injection and the orbital two stroke engine.

The two stroke enjoyed a period of use from about the mid-50s to the late 80s before going away. The big pipe that Alarumba refers to is an expansion chamber, which uses a resonant frequency to help scavenge exhaust gases.

The two stroke was reknowned for being "peaky" and having no torque, but in actual fact puts out a higher specific torque figure by capacity than a similar sized four stroke - it's just that the torque curve follows the horsepower curve, and this helps create the almost mythical "power band" that most of us old two stroke afficianados waffle on about.

There is now a "six" stroke engine in development, which is effectively a two stroke firing cycle with a valved head. Puts out a crap load of power from very small CC ratings.

**James Deuce** · 23rd November 2004, 15:41

Originally Posted by bungbung

So as to blend in with today's yoof, you will need to make certain one leg is rolled higher than the other depending on your eastside/westside allegiance

Wot abart us old school soufsiders?

**Storm** · 23rd November 2004, 15:57

Nice to see you sneaking up on your 2 star rating there Jim2

**Hitcher** · 23rd November 2004, 15:59

Originally Posted by bungbung

So as to blend in with today's yoof, you will need to make certain one leg is rolled higher than the other depending on your eastside/westside allegiance

I should have been a pair of ragged claws
Scuttling across the floors of silent seas.

**NordieBoy** · 23rd November 2004, 16:02

Originally Posted by bear

Yeah, good point on the google searching, but were most road bikes two stroke at one stage and then went to predominately four stroke engines?
Was interested as to why the more mature folk were asking the 2/4 stroke question.

Because it's the more mature folk who know the difference.
2-strokes are a simple engine and get more power per cc than 4-strokes.

A 4-stroke 250 is a nice little bike.
A 2-stroke 250 is an evil little monster.

In motorcross the 125 2-strokes race against 250 4-strokes and the 250 2-strokes race against 450 4-strokes.

**SPORK** · 23rd November 2004, 16:09

Excuse me! Only the oldies? Come on! As the most junior member here, I demand some respect! Don't think that only the oldies know everything (although I know nothing

)

**aff-man** · 23rd November 2004, 16:13

easy to tell difference if you hear ring-ning-ning-ning-ning you gots a 2-smoke and if it sounds like your mothers sewing machine you got a 4 stroke (250). plus i don't know of any 2-smokes bigger that 500cc does anyone else?????