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KnifeEdge_2K1
06-10-2004, 04:45 PM
how do 3 cylinder engines work? i know atleast some 3 cylinder engines use the same intake, compression, combustion, exhaust cycle as most regular petrol engines. at any given time in an inline or boxer engine each piston will be in one of the four cycles, no 2 pistons will be in the same cycle, this is so that the piston undergoing combustion can power the rest of the cylinders and the car itself, in a 3 cylinder the only method i can think of making it work is to let teh rotational enertia do the work which would theoretically push the idle speeds higher, does this make any sense?

Matra et Alpine
06-10-2004, 05:26 PM
how do 3 cylinder engines work? i know atleast some 3 cylinder engines use the same intake, compression, combustion, exhaust cycle as most regular petrol engines. at any given time in an inline or boxer engine each piston will be in one of the four cycles, no 2 pistons will be in the same cycle, this is so that the piston undergoing combustion can power the rest of the cylinders and the car itself, in a 3 cylinder the only method i can think of making it work is to let teh rotational enertia do the work which would theoretically push the idle speeds higher, does this make any sense?
You're right with the rotational inertia.
The more cylinders the more overlap the bang cycle has and the smoother the engine is.
Some of thsi can be taken out by increasing the mass of the crank with offset weights and increasing the flywheel. Unfortunately the increased mass makes for slower acceleration of rpm AND REDUCTION in top end revs as the higher mving mass becomes more unstable.
This smooths out the engine at lowe engine speeds, reduce the weight and the engine becomes temperamental and hard to keep at a low idle ( lightened BDAs and Climax's being some of the worst/lumpiest )

All engines run the Otto cycle in each piston.
What an engine designer CAN play with is the timing of each cylinders cycle by position the big end on the crank relative to erach other.
Some triples actually run like a twin with 2 cylinders firing together, others spread them evenly. BUT a triple is hard to keep balanced and counter-rotatiing balancing shafts are needed to keep things revving smoothly at high speeds. Few bikes still run triples and some of the japanese micro-cars of the 80s used 600cc triples - basically their bike engines !!
This timing can be cltered by the engine designer. In cars it has become pretty much standard and no vairation beyond the accepted norm for I, H and V configurations.
Bikes, however, are playing with this at the peak end of the sport. MotoGp bikes are run with 4s running as twins, opposing 3 and 2s in 5s and 4v1 has been suggested as having been tried by Honda. These make BIG differences in bikes due to their low weight and alter dramatically the way the bike can use the tyre. It seems this isn't tried in cars, probably because the extra weight and larger tyre contact makes it irrelevant.

foto_choppa
06-10-2004, 05:46 PM
wow i never realy thought about that, i honestly have no idea, great topic :D

KnifeEdge_2K1
06-10-2004, 07:44 PM
wow matra thankx, that cleared up alot, i kinda lost u at the end of the post but i think i got the gist of it :D

Matra et Alpine
06-11-2004, 03:22 AM
wow matra thankx, that cleared up alot, i kinda lost u at the end of the post but i think i got the gist of it :D
yeah, it rally needs a drawing :)

I'll try - 4 is easy ....

Firing order can be 1-2-3-4 or 1&3-2&4 so the first config gets one ignition and power pulse each revolution. The second gets one ignition every 2 revolutions but each power pulse is now twice as large. That assumes 180deg crank as the norm on a car. This is done to use the power pulse from one piston to be at the optimal timing for the other piston in compression. If this isn't done it becomes very hard to keep and engine inbalance as the compression piston has to then find the energy from some stored place - the crank or the flywheel.

In the Honda 5 it is secret what they are actually doing but those who've seen a crank have put forward a few suggestions....

One options would be 1-2-3-4-5, this could be achieved with a crank with 34degree offset. Like the 90deg-4 crank this is likely to be VERY unbalanced, need a large flyweehl and contra-rotatin balance shafts. What si THOUGHT they do is with a 180deg crank they fire 1&2&3-4&5, so get the 1 ignition per 2 revs and power pulses 2 and 3 times the single. They have been known to have tried the 'big bang' equivalent and had 4 sylinders and 1 working. In the bike world this has also been used on some tracks by Ducati with their twin firing BOTH at the same time. Bikes seem to be able to use tyres ( in a way I've never heard explained ) to 'smooth' out these pulses and to get better traction.

Engine design and timing is a real engineering challenge when you start playing with the cycle overlaps and timing. A search on the Hon RC211V engine and the Ducati V4 will likely bring up more details for those interested in what CAN be done :)

SilverArrowZ
06-12-2004, 06:49 AM
talk about 3 cylinder engine, 3 out of 10 cars at where i live is running on them :o and girls like them as they are small and cute.

Well, matra is really the encyclopedia here and i do not know much about how they work. All i know is that they idle at about 1200rpm, slightly higher than nomral I4 and V6 (no you won't be able to find V8 here).

These cars i refered to is actually local made, but all are made based on Daihatsu's chasis, mira, move... you get the idea.

the lowest displacement engine i got here at malaysia would be 660cc. Single cam 12 valve(4 per cylinder) and only gives out 31hp@6400rpm and yes you need an engine checkup after you bring 4 adult up a hill. best thing is you can get 50mile per gallon out of these "toys"