How does a 4-piston, 4 stroke internal combustion engine work?

[royp]

TL;DR Summary

What are the forces causing the piston movements of an internal combustion engine? And is there specific reasons for the synchronized piston movements viz. of P1 ,P4 and P2, P3?

Hi,
This post is about a 4-piston, 4 stroke internal combustion engine(ICE).Here is an youtube animation for such an engine.

I actually have 2 questions.
Q1: We have seen in demos and videos that in a 4-piston (ICE) engine, the pistons P1 ,P4 (pair 1) and P2, P3 (pair 2) are in phase when moving. While pair 1 and pair 2 are out of phase. Are there any specific reasons? Can we, instead, have the alternate ones, moving in phase (P1,P3 & P2,P4) and the adjacent ones with a phase difference?
Q2: In the 4-stroke (intake/compression/power/exhaust) reciprocating movements of the piston(s), what are the forces/thrusts causing the piston movement? For instance, in the power stroke, the piston is pushed down by the expanding hot air. But what forces the piston downward in the intake stroke (or the piston upward in the exhaust stroke)? Is it the momentum of the flywheel?
Many thanks in advance for the clarifications.

 

[jrmichler]

1) Try search terms internal combustion engine balancing to learn about this. Make sure you understand first order balance before trying to wrap your mind around second order balance. There most definitely are specific reasons for the crankshaft design where cylinders 1 & 4 move together and 180 degrees out of phase with cylinders 2 & 3.

2) Consider a one cylinder four stroke engine, such as a typical lawnmower engine. What keeps it running when power is generated for less than 1/2 of a revolution for every other revolution? Hint: Try search term internal combustion engine flywheel.

 

[royp]

Many thanks for your reply, @jrmichler. The pointers were very helpful!

 

[jack action]

Although there are no real advantages to changing the crankshaft design of an inline 4-cylinder configuration, some other engines can be available with different designs.

A V8 configuration is such an example. They can come in two configurations: flat-plane or cross-plane.

​

 

[royp]

Hi @jack action,
Many thanks for sending the brilliant link. Very insightful!

 

[Ranger Mike]

4 cylinder design - It is all about the design of the crankshaft. If you look at the inline 4-cyl engine, the crankshaft is a flat plane crank, meaning the throws are 180° out. With that, two cylinders are on one side of the crank and two cylinders are on the other side of the crank with five main bearings supporting it. The typical design of the four cylinder is to place cylinders 1 & 4 on one side and then 2 & 3 on the other side of the crankshaft. Since 1 & 4 always fall on the same side (pistons will be at top dead center (TDC) or bottom dead center (BDC) at the same time always), they cannot follow each other in the firing order. Due to this there can only be two different firing orders which work: 1-3-4-2 and 1-2-4-3.For Decades the 4 cylinder had 1243 I think because this was the Ford Model T 4 cylinder flat head firing order and it worked.The Ford NNA 1953 tractor was 4 cylinder and first overhead valve engine ( versus flat head) firing order was 1243Allis Chalmer 1243, Oliver, 1243, for Decades the 4 cylinder had 1243.The International Harvester introduced 4 cylinder firing order on IH Farmall ( I think) with 1342,Just about every other 4 cylinder engine has firing order 1342Ford, GMC, Mercedes, Nissan, Toyota.Why the change?IH was the first mass manufacturer to address the imbalance forces in a 4 cyl inline engine. See their C153 service manual. IH added the balancer shaft below crankshaft. It has to do with the fact that a piston moves a different distance in a specific degrees of crankshaft rotation at top dead center than that same piston does at bottom dead center. The piston travels a farther distance in the first and last quarter of crank rotation as compared to the second and third quarter of crank rotation. This is due to the side movement of the connecting rod. Picture a ladder leaning against your house. If you pull he bottom of the ladder away from the house the top of the ladder moves down yet the bottom of the ladder is still at ground level. In the first quarter of Crankshaft rotation it's pulling the piston down plus moving the bottom end of the rod away (from the house sort of speak) so there are two elements causing the piston to come down. In the second quarter of crankshaft rotation the crank is still pulling the piston downward but it's moving the rod (closer to the house) decreasing overall downward piston travel. Third quarter of crank rotation it's pushing the piston up but pulling the rod away from the house again, the fourth quarter of crank rotation the piston is going up and the rod is moved closer to the house.There are differences in harmonics in the crank shaft of 1243 engines versus 1342 engines. that do make a difference. The 1342 harmonics are more evenly spread over the crankshaft length. The vibration at the whole engine level might be the same, but internally 1342 has the advantage. Given the longevity benefit of a less harmonic engine ( and our Formula Car had a maxed out 4 cylinder and would vibrate your fillings out of your teeth at 7000 RPM) manufacturers went the 1342 route for performance and warranty.Performance - The amount of air that has to go through intake runners per revolution with 1243 is twice what will be needed for 1342. The amount of intake mixture is delivered in pulses either way. In 1342, the front port sucks then the rear port, then front, then rear.

In 1243, the front sucks twice in a row (sequentially) then the rear port sucks twice.
On a stump pulling tractor with maybe 2000 RPM max. air mixture does not really matter as you are chugging thru the farm fields at 1200 RPM. But on anything performance, you want a maximum intake charge so the firing sequence does matter greatly.

And this boys and girls is why the 4 cylinder Offenhauser Indy car engine was introduced in the 1930s with 1342 firing order! Still one of the winningest engines ever made!note - Flat 4 cylinder VW and Porsche design not included.

 

[jack action]

In 1243, the front sucks twice in a row (sequentially) then the rear port sucks twice.

That is still true with 1342, which could be rewritten as 2134 (twice in a row at the front followed by twice in a row at the rear).

 

[Mark44]

An unusual setup for a 4-piston engine was the Ariel Square 4, a British motorcycle produced between 1931 and 1959. It had two crankshafts, making it effectively a joined pair of inline two-cylinder engines in a single engine case and a single head. Early versions had overhead cams, but later versions ran overhead valves.

Here's a picture of a '54 Ariel I saw at an antique motorcycle show about a week ago.

 

[Ranger Mike]

That is still true with 1342, which could be rewritten as 2134 (twice in a row at the front followed by twice in a row at the rear).

Jack!
As usual the Master is correct. Your linear thinking is at a level I hope to accomplish someday.
I took another look at intake flow and the main concern is maximum flow in the intake runners. There is just not a big difference.

 

[royp]

Many thanks, Ranger Mike for a very detailed and thorough explanation! And same to you, Mark44 for highlighting the unusual case for the motor cycle!

 

[Janus]

Oddly enough, I've just been working on making animations for both a V-8 and a radial engine. The radial engine so far:

 

[Mark44]

Oddly enough, I've just been working on making animations for both a V-8 and a radial engine.

Since you mentioned it, my machinist buddy, who helped me put together a 1945 Harley flathead engine, is building a scale model 18-cylinder radial aircraft engine, nearly 100% from scratch. He bought the plans from the engine designer, and with the plans is a cast aluminum piece that is the front of the engine. Everything else (except the spark plugs) -- pistons, valves, cylinders, heads, rockers, pushrods, oil pump, etc., he will make. When it's finished, I think it will be about 16 to 18 inches in diameter.