Engine technology thread.

[serialk11r]

Okay I know what you mean, the flame takes longer to spread and so the fuel takes longer to burn if the chamber is flatter, so that goes to my last question again, why can't they use more spark plugs? Do they take up too much room?

lol @ above post.
@ Snaps assuming a relatively simple model for conflagration, it's probably not very hard to model the combustion chamber and figure out how to get good, complete burns. Obviously there are other problems to deal with however when thinking of designing the engine such as frictional loss, which is probably greater if you have a longer stroke and narrower bore, and of course stress on the components due to higher piston speed. I imagine gasoline burns so quickly that under many circumstances this doesn't really make a big difference.

[Snaps]

Originally Posted by Dimman

Ow.

??

Are related rates of change not calculus? As far as I have learned them they are (1st year Uni)...

Originally Posted by serialk11r

Okay I know what you mean, the flame takes longer to spread and so the fuel takes longer to burn if the chamber is flatter, so that goes to my last question again, why can't they use more spark plugs? Do they take up too much room?

lol @ above post.
@ Snaps assuming a relatively simple model for conflagration, it's probably not very hard to model the combustion chamber and figure out how to get good, complete burns. Obviously there are other problems to deal with however when thinking of designing the engine such as frictional loss, which is probably greater if you have a longer stroke and narrower bore, and of course stress on the components due to higher piston speed. I imagine gasoline burns so quickly that under many circumstances this doesn't really make a big difference.

Though so Just thinking to myself though It wouldn't surprise me if they did do something like what I said, but I wouldn't hesitate to admit that it's likely far more compicated than I imagine.

[old greg]

Originally Posted by serialk11r

it's probably not very hard to model the combustion chamber and figure out how to get good, complete burns.

Modelling the combustion chamber is easy. Modelling the combustion process is not easy at all, especially if you want the results to mean anything. But this is always done, and car companies employ some very bright people who do little else.

[serialk11r]

So obviously you'd have to consider fluid dynamics and that makes things VERY complicated, but companies have a good deal of experience with this, and for many applications (low rpm) I'm guessing you can assume the gas burns fast enough for it to not matter as much that the combustion chamber be really really well tweaked.

[old greg]

You can't really assume that at all. Low port velocities make for poor fuel atomization and inefficient combustion. I mean sure, any old thing will probably run but it isn't necessarily going to be efficient. Fuel consumption is a huge concern these days, and most cars spend the majority of their lives at low rpm. So engine designers spend a lot of time designing pistons, combustion chamber, ports, valves etc. to improve combustion efficiency at low rpm.

There has been a huge amount of time and money spent over the past 3+ decades developing accurate mathematical models of the combustion process. Teams of guys with doctorates in computational fluid dynamics have developed, validated and refined these models and put them in computer program form. It's a very complicated subject, and you won't see anyone without at least a master's degree in Engineering or Physics working on this stuff at a major car company, but a lot of the heavy lifting is done by computers (which is why any of this is possible).

[serialk11r]

Okay guess I was wrong, thanks for the info

But still going back to my original question, why can't they use another spark plug like on Mazda rotaries? Isn't that how they alleviate some of the inefficiency caused by a very long combustion chamber? What's the drawback to that besides a small increase in cost and taking up a tiny bit more room in the head?

[Allch Chcar]

Originally Posted by serialk11r

Okay guess I was wrong, thanks for the info

But still going back to my original question, why can't they use another spark plug like on Mazda rotaries? Isn't that how they alleviate some of the inefficiency caused by a very long combustion chamber? What's the drawback to that besides a small increase in cost and taking up a tiny bit more room in the head?

If you're still interested in the narrow bore and two spark plug questions. The answer is the most efficient setup is a narrow bore with two spark plugs. One of the things Honda did with their L series is build a wide angle head with 2 valves so they could fit two spark plugs on a 73mm bore. In that case they went to extraordinary lengths to find the ideal combustion for low-mid RPM operation.

Temple of Vtec Asia L series

The multi spark plug can help for bigger bores. Ford even had 4 plug versions of the Pinto/Lima 4 banger back in '93 and those were low RPM torque biased but they were not as efficient on fuel as they could have been.

[serialk11r]

That's pretty cool, so I take it the problem with implementing multiple spark plugs is mainly finding space for the plugs...

[Exage]

Originally Posted by Dimman

Engine predictions:

FB20 standard bore and stroke:
84mm bore X 90mm stroke: 200 bhp @ 7000 rpm, redline 7500 rpm (also redline of the Mk1 concept's interior, so...)
But not too mod-friendly.

FB25/20 de-stroked fantasy:
94mm bore X 72mm stroke: 205 bhp @ 7500 rpm, redline 8500 rpm
Very mod-friendly.

I found something interesting when calculating the FB25 de-stroked engine.

We de-stroke it by 18mm which means the piston is sitting 9mm lower in the cylinder at TDC (,9mm higher at BDC) with a 129.3mm rod. A 129.3mm rod to 72mm stroke is 1.796 (higher than K20, 2ZZ: 1.62).

Stock CR was 10.5:1 leaving 9.4737mm between piston and head with a 90mm stroke. With a 72mm stroke it would need 7.5790mm between piston and head to maintain 10.5:1 CR.

This is of course without lowering the FB25 deck height, would mean that it would need a 10.89mm longer rod then the 129.3mm to maintain 10.5CR on a 72mm stroke resulting in a 1.947 rod-stroke. Might be a good custom race engine (it would be better with a semi/closed deck and) with the appropriate work on the heads and timing.

So it could be a FB20. Unless they decreased the deck height... Any thoughts?

[Dimman]

Originally Posted by Exage

I found something interesting when calculating the FB25 de-stroked engine.

We de-stroke it by 18mm which means the piston is sitting 9mm lower in the cylinder at TDC (,9mm higher at BDC) with a 129.3mm rod. A 129.3mm rod to 72mm stroke is 1.796 (higher than K20, 2ZZ: 1.62).

Stock CR was 10.5:1 leaving 9.4737mm between piston and head with a 90mm stroke. With a 72mm stroke it would need 7.5790mm between piston and head to maintain 10.5:1 CR.

This is of course without lowering the FB25 deck height, would mean that it would need a 10.89mm longer rod then the 129.3mm to maintain 10.5CR on a 72mm stroke resulting in a 1.947 rod-stroke. Might be a good custom race engine (it would be better with a semi/closed deck and) with the appropriate work on the heads and timing.

So it could be a FB20. Unless they decreased the deck height... Any thoughts?

Originally Posted by sneaky

Here's some info on what Subaru is planning for the WRX & STi models

http://www.caradvice.com.au/116515/s...-from-wrx-sti/

This is interesting, we've been speculating a bit on this already:

Speaking to CarAdvice in New York, Subaru’s Impreza Product General Manager, Mr Akihide Takeuchi, said work on the replacement turbocharged engine has commenced but would not give any further details. He did confirm that it will not be based on the EJ25 and will not simply be a turbocharged version of the new boxer engine.

What do you think of this?

[old greg]

Originally Posted by Dimman

What do you think of this?

I see three possiblilites.

1) It'll be a destroked/bored FB like we were talking about, with slightly different heads. A 72mm stroke is a pipe dream though IMO, it won't be lower than the EJ20's 75mm stroke as Subaru is looking for higher torque/fuel economy.

On the plus side, they may want to stay away from the asymmetrical rods on their boosted engines which would keep stroke reasonable. ~80mm seems likely (ej25 = 79mm), and is plenty low for some very high rpms. Remember that the F20c had a stroke of 84mm and revved to 9000 rpm.

With the same deck height as an FB20, an 80mm stroke would allow 134.3mm rods. At 8000rpm that means a mean piston speed of 4200 ft/min and peak piston acceleration of 3718 g, which ought to be fairly reliable (compare that to 4724 ft/min and 4344 g for a normal FB20).

Figure a forged bottom end (that's just how Subaru rolls), probably a semi-closed deck, with forged pistons in the STi and cast in the WRX.

2) Something similar to #1 but with 2.5L, ~85mm stroke

3) They go for max. displacement. The FB kept the 113mm bore pitch of the EJ, which means that there should be enough room for a 99.5mm bore (EJ25). At 99.5mmx90mm we get a 2.8L engine. This doesn't seem all that likely with the CAFE deadline coming, but you never know.

[ichitaka05]

Originally Posted by old greg

I see three possiblilites.

1) It'll be a destroked/bored FB like we were talking about, with slightly different heads. A 72mm stroke is a pipe dream though IMO, it won't be lower than the EJ20's 75mm stroke as Subaru is looking for higher torque/fuel economy.

On the plus side, they may want to stay away from the asymmetrical rods on their boosted engines which would keep stroke reasonable. ~80mm seems likely (ej25 = 79mm), and is plenty low for some very high rpms. Remember that the F20c had a stroke of 84mm and revved to 9000 rpm.

With the same deck height as an FB20, an 80mm stroke would allow 134.3mm rods. At 8000rpm that means a mean piston speed of 4200 ft/min and peak piston acceleration of 3718 g, which ought to be fairly reliable (compare that to 4724 ft/min and 4344 g for a normal FB20).

Figure a forged bottom end (that's just how Subaru rolls), probably a semi-closed deck, with forged pistons in the STi and cast in the WRX.

2) Something similar to #1 but with 2.5L, ~85mm stroke

3) They go for max. displacement. The FB kept the 113mm bore pitch of the EJ, which means that there should be enough room for a 99.5mm bore (EJ25). At 99.5mmx90mm we get a 2.8L engine. This doesn't seem all that likely with the CAFE deadline coming, but you never know.

Sorry, call me "jackass", "too damn picky" or whatever you want... but it's STI now and not STi anymore.

The Subaru Impreza WRX also comes in a high performance STI edition (formerly designated "STi" before the 2006 model year), designed by STI

http://en.wikipedia.org/wiki/Subaru_..._International

Sorry for off topic, but it really bugs me when I see that.

[old greg]

Stop being so damn picky, you jackass.

[tranzformer]

I am itching for something official from Toyobaru.