Diesel Engine Working Principle

Thread Starter

RdAdr

Joined May 19, 2013
214
The four steps are like this:

1) The piston starts at the top, the intake valve opens, and the piston moves down to let the engine take in a cylinder-full of air and gasoline. This is the intake stroke. Only the tiniest drop of gasoline needs to be mixed into the air for this to work. (Part 1 of the figure)

2) Then the piston moves back up to compress this fuel/air mixture. Compression makes the explosion more powerful. (Part 2 of the figure)

3) When the piston reaches the top of its stroke, the spark plug emits a spark to ignite the gasoline. The gasoline charge in the cylinder explodes, driving the piston down. (Part 3 of the figure)

4) Once the piston hits the bottom of its stroke, the exhaust valve opens and the exhaust leaves the cylinder to go out the tailpipe. (Part 4 of the figure)

My questions:
1) What opens the intake valve? What moves the piston down?
2) What moves the piston back up?
3) Now I see that the piston moves down because of the explosion.
4) What moves the piston back up? And what opens the out valve?
 

joeyd999

Joined Jun 6, 2011
4,291
The four steps are like this:

1) The piston starts at the top, the intake valve opens, and the piston moves down to let the engine take in a cylinder-full of air and gasoline. This is the intake stroke. Only the tiniest drop of gasoline needs to be mixed into the air for this to work. (Part 1 of the figure)

2) Then the piston moves back up to compress this fuel/air mixture. Compression makes the explosion more powerful. (Part 2 of the figure)

3) When the piston reaches the top of its stroke, the spark plug emits a spark to ignite the gasoline. The gasoline charge in the cylinder explodes, driving the piston down. (Part 3 of the figure)

4) Once the piston hits the bottom of its stroke, the exhaust valve opens and the exhaust leaves the cylinder to go out the tailpipe. (Part 4 of the figure)
This is not a description of a diesel engine!
 

ISB123

Joined May 21, 2014
1,238
1.Starter motor starts the engine by cranking the crankshaft which results in mixture getting sucked in the cylinder and being ignited by compression.Camshaft controls the valves.
2.Inertia and firing sequence of pistons.
3.
4. Firing sequence of pistons. E.g. 4 cyl. engine firing cycle 1-3-4-2.
Diesels don't use spark plugs since they depend on heat generated by compression to ignite the mixture.

 

ian field

Joined Oct 27, 2012
6,539
The valves are controlled by the camshaft which is driven by the crankshaft.
Apparently some manufacturers are developing solenoid operated valves - but I doubt any are in production yet.

It may have been Honda that released a bulletin about an engine that can switch off some of the cylinders for economy mode.

AFAIK: they plan to leave the valves closed on the resting cylinders - Some of the energy gets returned on the downstroke if you compress the gas in the cylinder, the inertia/resistance of shifting cylinder gas through open valves would consume significant energy.
 

Alec_t

Joined Sep 17, 2013
10,746
an engine that can switch off some of the cylinders for economy mode.
I don't think I'd buy one. I'd expect increased vibration hence increased engine wear and shorter engine life. Good for car company profits maybe, but hardly economy for the motorist.
 

Thread Starter

RdAdr

Joined May 19, 2013
214
Thanks for answers. I also found this video:

So the starter motor provides the initial momentum and then the pistons move by themselves due to inertia and air-fuel explosions.

I need to read more on this. I have very basic knowledge in engines.
 
Last edited:

#12

Joined Nov 30, 2010
18,186
I don't think I'd buy one. I'd expect increased vibration hence increased engine wear and shorter engine life. Good for car company profits maybe, but hardly economy for the motorist.
I think you're about a century behind in your understanding of technology. Design them right and you find the observed fact of diesel engines out-lasting gasoline engines by a factor of 2 to 3.
 

jpanhalt

Joined Jan 18, 2008
8,510
In reference to Ian Field's comment about switching off cylinders:

I don't think I'd buy one. I'd expect increased vibration hence increased engine wear and shorter engine life. Good for car company profits maybe, but hardly economy for the motorist.
Actually, they are becoming quite common. Ford has some models, and the new $100,000 Corvette switches off 4 of 8 cylinders at any one time, unless you stomp on it. Then it really goes.

John
 

joeyd999

Joined Jun 6, 2011
4,291
It may have been Honda that released a bulletin about an engine that can switch off some of the cylinders for economy mode.
This is true. The 'Eco' mode in my Odyssey seamlessly knocks out 3 of 6 cylinders when power is not required, and seamlessly reengages them when it is. All I see is the indicator light going on and off...I've never felt a thing.
 

ian field

Joined Oct 27, 2012
6,539
This is true. The 'Eco' mode in my Odyssey seamlessly knocks out 3 of 6 cylinders when power is not required, and seamlessly reengages them when it is. All I see is the indicator light going on and off...I've never felt a thing.
I think cylinder bank isolation was already around, that simply removed a whole crankshaft serving one bank of cylinders from the drive train.

Disabling the valves on some cylinders is a newer idea and I don't know how far its developed so far.
 

crutschow

Joined Mar 14, 2008
24,130
I don't think I'd buy one. I'd expect increased vibration hence increased engine wear and shorter engine life. Good for car company profits maybe, but hardly economy for the motorist.
Why would you expect more vibration?
Most engine vibration is cause by the imbalance of the rate the pistons are accelerated up and down (especially in four cylinder in-line engines), not by whether they are firing or not.
 

ian field

Joined Oct 27, 2012
6,539
Why would you expect more vibration?
Most engine vibration is cause by the imbalance of the rate the pistons are accelerated up and down (especially in four cylinder in-line engines), not by whether they are firing or not.
It certainly has an effect - as I discovered on a 4-cyl motorcycle with one fouled plug.

It probably has more to do with power delivery than the reciprocating mass of pistons etc, but it doesn't do the engine any good.

Presumably the manufacturer would take these things into account and design carefully.
 

Alec_t

Joined Sep 17, 2013
10,746
I think you're about a century behind in your understanding of technology.
Oh, at least a century :).
Why would you expect more vibration?
Because I've experienced increased vibration on a 4-cylinder in-line petrol engine with a blocked injector. Perhaps if two injectors had been blocked I wouldn't have noticed ;). I can see that 3 out of 6, or 4 out of 8 cylinders non-firing would have a better chance of balanced running.
 

crutschow

Joined Mar 14, 2008
24,130
It certainly has an effect - as I discovered on a 4-cyl motorcycle with one fouled plug.

It probably has more to do with power delivery than the reciprocating mass of pistons etc, but it doesn't do the engine any good.

Presumably the manufacturer would take these things into account and design carefully.
Note that a cylinder with a fouled plug is different than deliberately turning off a cylinder.
In the latter case they keep the valves closed so that you aren't pumping air through the cylinder.
Not sure if that would make a difference in the vibration though.
It's likely that turning off more than 1 cylinder at a time is what minimizes any added vibration, as Alec noted.
 

WBahn

Joined Mar 31, 2012
25,115
The idea of effectively removing cylinders from the engine is decades old. I remember see car commercials from the 70's or 80's where they touted how the car would reduce the number of cylinders from 8 to 6 to 4 to achieve really high fuel economy. Since it seemed to be a passing fad, I concluded that the increases in economy, if any, were marginal and the increase in maintenance or initial investment outweighed any savings.
 
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