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[STMO123]

When a cambelt lets go, the engine is seiously, sometimes terminally, damaged. This is due to the pistons 'interfering' with the valves. Hence the phrase interference engine.

On a 'non-interference' engine, this does not happen, as the piston does not travel far enough up the bore to do any damage.(Or the valves not far enough down?)

Why aren't all engines non-interference? What are the advantages/disadvantages of the two set-ups?

Ta.

[hotel21]

Number of factors.  

Compression ratio, valve diameter, number of valves, valve opening lengths, timing and duration of valve opening/closing relative to piston movement.

The higher the performance or output of the engine, probable higher risk of damage.

[STMO123]

Number of factors.  

Compression ratio, valve diameter, number of valves, valve opening lengths, timing and duration of valve opening/closing relative to piston movement.

The higher the performance or output of the engine, probable higher risk of damage.

Aye. I know what factors are involved Broocie. But what advantage is gained by shortening the length of the bore, thus causing the piston/valve chat?

[STMO123]

It's OK. I've sat and scratched my arse/picked my nose for a few minutes and I've worked it out

[hotel21]

It's OK. I've sat and scratched my arse/picked my nose for a few minutes and I've worked it out

Have a skeg at this page.  Loads of info as regards diesel versus petrol as well as some sums to work out efficiences, but it also explains about under/over square engines, ie the ratio of bore diameter to stroke and why its important.  Piston speed plays a significant part, hence short stroke high revs in cars and racing engines against long stroke low revs in the marine diesel environment.    

[STMO123]

It's OK. I've sat and scratched my arse/picked my nose for a few minutes and I've worked it out

Have a skeg at this page.  Loads of info as regards diesel versus petrol as well as some sums to work out efficiences, but it also explains about under/over square engines, ie the ratio of bore diameter to stroke and why its important.  Piston speed plays a significant part, hence short stroke high revs in cars and racing engines against long stroke low revs in the marine diesel environment.    

Cheers Broocie

[Golfbuddy]

It's OK. I've sat and scratched my arse/picked my nose for a few minutes and I've worked it out

Have a skeg at this page.  Loads of info as regards diesel versus petrol as well as some sums to work out efficiences, but it also explains about under/over square engines, ie the ratio of bore diameter to stroke and why its important.  Piston speed plays a significant part, hence short stroke high revs in cars and racing engines against long stroke low revs in the marine diesel environment.    

Just spent 20 minutes reading this. I'm none the wiser and I am now seeing double.  

[Markjay]

It's OK. I've sat and scratched my arse/picked my nose for a few minutes and I've worked it out

Have a skeg at this page.  Loads of info as regards diesel versus petrol as well as some sums to work out efficiences, but it also explains about under/over square engines, ie the ratio of bore diameter to stroke and why its important.  Piston speed plays a significant part, hence short stroke high revs in cars and racing engines against long stroke low revs in the marine diesel environment.    

Just spent 20 minutes reading this. I'm none the wiser and I am now seeing double.  

You're the clever one amongst us... if you can't figure it our then I am not going to even try!  

[Markjay]

On my Mk1 Astra (1.8 OHC Petrol) I had the belt free-revving after it got disconnected from the crank pulley, I was on the Motorway at the time and didn't realize what happened other than the engine went quite, actually tried re-strating the engine by pressing and depressing the clutch so the bottom end (and pistons) was definitely spinning, but suffered no damage, and in fact I was able to fix the problem on the hard shoulder and continue my Journey (timing a 4-pot 8-valve engine is child's play compared to these modern beasts...)

On the other hand a Ford Escort 1.8 Diesel I was driving had the timing belt snapped at parking speed, I switched off immediately but the engine was scrapped.

I think that as said this has to do with higher compression ratios and also with less space due to 4-valves per cylinder design.

[Golfbuddy]

It's OK. I've sat and scratched my arse/picked my nose for a few minutes and I've worked it out

Have a skeg at this page.  Loads of info as regards diesel versus petrol as well as some sums to work out efficiences, but it also explains about under/over square engines, ie the ratio of bore diameter to stroke and why its important.  Piston speed plays a significant part, hence short stroke high revs in cars and racing engines against long stroke low revs in the marine diesel environment.    

Just spent 20 minutes reading this. I'm none the wiser and I am now seeing double.  

You're the clever one amongst us... if you can't figure it our then I am not going to even try!  

Are calling me a smart arse?  

 

[STMO123]

To put it simply, I THINK:

When a piston reaches TDC there is space between the top of the piston and the top of the cylinder bore. Lets say this space is 100cc. The engineers (yes, Mark, engineers) have worked out how much fuel/air is required in this space so that, when ignited, will give the necessary BANG to drive the piston down at the required speed (power)

If this space is increased to, say, 200cc, then to produce the same power there would have to be twice the amount of fuel/air. There goes your fuel ecomony. Imagine an omega that did 14mpg!

So, I guess, it'a fuel to power output ratio thingy that governs the gap and, therefore, interference/non-interference.

[miggy]

When a cambelt lets go, the engine is seiously, sometimes terminally, damaged. This is due to the pistons 'interfering' with the valves. Hence the phrase interference engine.

On a 'non-interference' engine, this does not happen, as the piston does not travel far enough up the bore to do any damage.(Or the valves not far enough down?)

Why aren't all engines non-interference? What are the advantages/disadvantages of the two set-ups?

Ta.

Not a daft question, i would be interested to know this, good question steve, I wait in antipitation for feedback.

 

[STMO123]

Well....look above your post

[miggy]

It's OK. I've sat and scratched my arse/picked my nose for a few minutes and I've worked it out

Have a skeg at this page.  Loads of info as regards diesel versus petrol as well as some sums to work out efficiences, but it also explains about under/over square engines, ie the ratio of bore diameter to stroke and why its important.  Piston speed plays a significant part, hence short stroke high revs in cars and racing engines against long stroke low revs in the marine diesel environment.    

Just spent 20 minutes reading this. I'm none the wiser and I am now seeing double.  

Take more water with your whiskey.

 

[Marks DTM Calib]

Interesting and easy ish to answer.

To get optimum combustion and hence efficiency and power in a petrol engine you idealy want.

1) Ideal air/fuel mix
2) Optimum spark timing
3) Highest possible compression ratio
4) Optimised combustion chamber shape
5) Minimal resistance to gases through inelt and exhaust system (volumtric efficiency)

1 and 2 are looked after by the management system but 3, 4 and 5 are all down to the physical design of the power plant.

Also consider that the stroke v bore ratio is important to i.e.

An engine which has a stroke longer than bore diameter is considered to be a long stroke engine, this will ahve lots of low down grunt but, wont rev so freely (see the above article regarding piston speed etc)

An engine which has a stroke shorter than the bore diameter is considered to be a short stroke engine, this will have less low down torque but will rev much higher (i.e. F1 engines and most modern bike engines).

The ideal for most cars is what is described as a square engine where the stroke is approx equal to the bore.....this gives reasonable low down torque and still maintains a reasonably free reving engine and this is what most of the Omega engines are......

Then consider the fact that to get say a 10:1 compression ratio  with a stroke of say 79.6mm (as per the 2.5 V6) the space between the piston crown and the head at TDC is only 7.96mm!!!

Given that the valve opening is 8mm and that the valves are set at an angle to allow 4 per cylinder to be fitted to get better volumetric efficiency and that the head is not flat and has a combustion chamber scalped into it etc etc etc....you can see its not easy to get a non-interference engine if you want all the desirable characteristics demanded of a modern power plant!

And the Ford diesel going bang is no surprise as it was a derivative of the CHV (if my memory serves me correctly) which in petrol form used to bend valves for a living to.