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Any advantage to a internal combustion engine that fires two pistons simultaneously?

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arnhalen
#1
Nov27-12, 09:06 PM
P: 2
Aloha,

This is my first post, I am happy to be here.

Would there be any advantage to a internal combustion engine(ICE) that fired two pistons simultaneously? This would be an alternate-firing ICE that would fire the pistons in pairs every 180, instead of one piston fire and the other piston in the intake stroke every 90.

For example an alternative-firing small block chevy(ALTICE) 350 cubic inch engine. In the ICE at 0 crankshaft, pistons 1 and 6 are at the top of the cylinder. Piston 1 fires (Power-stroke) and piston 6 is in the (intake-stroke). ALTICE in comparison, has both pistons 1 and 6 that are in the (power-stroke), this 1-6 pair would have its intake-stroke on the second rotation of the crankshaft. ALTICE would fire the pistons 7-4 at the 180. On the second rotation of the engine cycle pistons 5-8 fire at 450 and 3-2 fire at 630. See attached image.

ALTICE would be like converting the SBC V8 into (2) four-cylinder engines that fires "in-phase" utilizing the chevy cross-plane crankshaft. In contrast to a Ferrari 458 V8 engine that is like (2) four-cylinder engines that fire "out-of-phase" using a flat-plane crankshaft.

What would the pro and cons of an ALTICE type engine? Would the torque be increased because of the second pistons simultaneous firing? Would the piston (power) pulse increase? Would the fuel consumption be the same? The same 8 pistons fire over the same two crankshaft rotations in both the ICE and the ALTICE. Do you think that the ALTICE would rev easier because of the every 180 crankshaft firing, in comparison to ICE?

What would be the disadvantages of an ALTICE design? To my knowledge there is no ICE that fires two pistons simultaneously, I was just wondering why.
Attached Thumbnails
ALTICE.jpg  
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Ranger Mike
#2
Nov28-12, 04:19 AM
Sci Advisor
PF Gold
P: 1,478
welcome....good question...let us review the current firing patterns..a 4 cylinder fires a piston every 90 degrees.. a V8 fires every 45 degrees, a V10 fires every 36 degrees..the V10 is very vibration free compared to the 4 cylinder..we race a 4 cylinder and it is one big vibration monster...stress on all major components...firing every 180 degrees would be extremely stressful on crankshaft, all components and engine would have high customer complains regarding ride comfort,,,plus HP would have to be down I would think...
xxChrisxx
#3
Nov28-12, 04:28 AM
P: 2,055
Damn. Supid internet lost my post, I'll just type out a summary.

It is technically possible to have two pistons fire at the same time, but there is no real advantage to it. You'll lose smoothness and increase durabiliy issues.

Imagine this as a roundabout in the park with two handles and two people pushing. To make the best ride possible, they push as hard as they can. But each needs to rest for 1 revolution after pushing.

So you have 2 options:
a: 1 push per revolution with alternating people. 1-1-1-1-1-1.
b: 2 pushes with a rest revolution. 2-0-2-0-2-0.

Both opions will give the same average torque and rotaional speed. Option A, will give a far smoother ride, less jerk from a big push, and less speed variation.

Person 1 and 2 will also need better communication for option b making it more complicated. If one pushes a little bit too early, the roundabout will start to speed away from them making the second push less effective.


This is why you don't see a V8 with 2 simulaneous firing. A 4-pot with te same capacity will be more efficient, and a V8 with a power stoke overlap will be far smoother.

HowlerMonkey
#4
Nov28-12, 12:12 PM
P: 277
Any advantage to a internal combustion engine that fires two pistons simultaneously?

There have been a few "big bang" inline 4 cylinder racing engines which fire the two outer cylinders and then the two inner cylinders.

This is what it sounds like in a normally aspirated car.

http://www.youtube.com/watch?v=7BSKTooB4TY

The most extreme "big bang" big bang firing order engine is more of a "quasi-big bang" as used in Valentino Rossi's championship winning R1.

What they did was crowd the combustion events within the first 180 degrees of the 720 degree 4 stroke cycle to reduce the amount of distinct power pusles.

Apparently, this helps traction on race bikes.

Yamaha also went further and split the crankpin angles slightly to produce a "twingle" which slightly separates the two cylinder's firing by a few degrees to overlap the firing events.
jack action
#5
Nov28-12, 08:37 PM
P: 578
Quote Quote by arnhalen View Post
What would the pro and cons of an ALTICE type engine?
As said by others, having one big pulse or two successive small ones will give the same torque output in the end. So neither a pro or con on this matter.

For balance, well the mechanical components are the same, so not much changes there either.

The uneven firing throughout the 4 strokes will increase vibrations and stresses, as others mentioned, and might also required a bigger flywheel to even them out (= lower rpm acceleration).

But I see one characteristic that might be an advantage: the exhaust stroke. On the ALTICE engine you've shown, you have 4 even-fire cylinders on each cylinder bank - just like a single-plane V8 - although recurring every two revs. I'm guessing that, with an appropriately tuned exhaust system, you could take an advantage from that to get slightly more power (but not better fuel economy, though).

You might be interested in this very nice video from yamaha that explains the advantages of the crossplane inline-4 engine (which is pratically the same as half your engine; IMHO, yours might even be better with the firing order):

arnhalen
#6
Nov29-12, 04:00 AM
P: 2
I appreciate everyone's input. I did not even know there was a thing as a "Big-Bang" engine for motorcycles. Interesting subject. Thanks again everyone.
-arnhalen
Averagesupernova
#7
Nov30-12, 09:00 AM
P: 2,537
Horizontally opposed two-stroke engines have been set up to fire at the same time. One would think it actually will improve balance and vibration compared to the alternative.
Lsos
#8
Jan18-13, 02:59 AM
P: 777
Quote Quote by Averagesupernova View Post
Horizontally opposed two-stroke engines have been set up to fire at the same time. One would think it actually will improve balance and vibration compared to the alternative.
I was wondering if the same is true for horizontally-opposed four-stroke engines, such as those seen in Subarus. They are supposed to be perfectly balanced....I imagine the only way to do this is to fire them simultaneously?


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