Why do SV's have engine braking?

[ophic]

Originally Posted by streetos

What!
Diesels have a lot more engine braking because they don't generally have a throttle valve and are therefore pumping a lot of air.

If you had ever worked with vacuum pumps you would know that the pump loads up when pumping the air out and off loads as it reaches full vacuum. If you don't believe this, try it on your vacuum cleaner. Put your hand over the suction pipe while it's running. Does the motor slow down or speed up? Explain.

sorry utter cobblers. In the case of a diesel most of what is lost compressing the air is regained when it expands. Simple air pumping effect. Argue if you like - some diesels are deliberately fitted with compression brakes to make the character of the engine on overrun more similar to petrol. I don't claim to be an expert but some principles are really rather simple. Wouldn't expect may people to agree with you here.

Just bear in mind we are trying to compare like with like. So i'm comparing something like a large capacity torquey petrol engine with a diesel here. If you produce a petrol engine with the torque characteristics of a diesel i would expect the engine braking to be massive.[

[ophic]

anyway Skidmarx has answered all the questions i had in his excellent post above. I really can't be bothered with this thread anymore as engine theory always seems to induce a lot of b*llo*x from ppl.

Bye

You've gotta be takin the ****.

Originally Posted by skidmarx

This is all a red herring! It is to do with the size of the bore. This directly relates to maximum piston speed. There is an upper limit to this as the oil metal boundary breaks down with excess speed. In order to limit the piston speed you can either:
1 increase the number of cylinders, this will reduce stroke and hence piston speed. (6 cylinder honda engines?)
2 Shorten the stroke of the engine.
Both of these reduce the piston speed as the piston has a shorter distance to travel. Manufacturers exploit this by making shorter stroke engines and IL4's as this allows them to rev higher. Power=TorqueXspeed
SO........big cylinders have restricted to the number of revs they can achieve without breaking down the oil metal boundary. Slower revving means different vavle timings, see earlier for why that makes a difference!

omg who learnt you engineering priciples

Shorten the stroke = a lower rpm maximum

no offence but when i pick myself of the floor and stop laughing i might inject some sense into the conversation

so what your saying is that a boat with a 40 foot stroke has an rpm of 10.000.000 wow interesting concept and there i was thingking the max rpm of a boat was about 800

bore x stroke does in fact determine the max rpm of an engine and its when the engine goes under square the the rpm will raise

by under squre i mean bore = 50 stroke = 49 for instance

if you go oversquare longer stroke this will infact turn the bike into what is commonly known as a thumper longer stroked engines vs bore size are more plodders than thrashers

a typical race engine in a car will have a smaller stroke than piston diameter in order to reach high end rpms bike do tend to follow a similar format


take the motorcrossers and thumpers of the tracks

end of rant

[ophic]

i thought the terms over and under square were used the other way round. Anyway - aren't you both saying that a shorter stroke will increase revs? He said it will reduce the piston speed - which it will. Or alternatively keep max piston speed the same and increase the revs.

And wasn't I gonna butt out of this thread?

sorry barty boy.....I think I'm right! Short stroke refers to the throw of the con rod. The crucial thing here is the piston speed. This does depend of the length of the stroke honest....

Oh and I'm with ophic on his appraisal of the situation so far

Originally Posted by skidmarx

sorry barty boy.....I think I'm right! Short stroke refers to the throw of the con rod. The crucial thing here is the piston speed. This does depend of the length of the stroke honest....

but piston speed directly relates to rpm

a short throw conrod and a square configuration will produce a higher rpm and there fore piston speed than a long throw rod engine of the same cc

sorry but that how it works on real engines

It goes without saying that piston speed directly relates to rpm. However, that's not the whole story. Take 2 engines both say....650 cc? One long stroke, ie small bore but a longer throw on the crank, and the other 'over square' with a large bore and short throw of the crank. In your world for any given rpm, the piston speed would be the same...but alas no...The long throw engine would have a significantly higher maximum piston speed! The piston speed is not constant through the stroke, but rather varies sinusoidily according to theangle of the crank. Think of it this way, in a long stroke engine the piston has further to go in a given time than with a short stroke engine.

yes i understand exactly what you are saying there but you are saying that a long stroked engine will have a higher rpm max which is wrong it wont

your getting into an inertia debate now which is different or was that what you were trying to get at before ??????