Engine theory 4 stroke- smart people needed

[ROBSREDFORD]

Jay or other admins delete if not allowed. I  really don’t have another place to ask and I know there are some smart folks on this board

Diesel Engine I’m working on runs of factory written file that triggers injection based off cam position sensor and crank position sensor   Happens to be Caterpillar 3400/c15/c18 series

Constants are-
Deck height, camshaft, all gear train, connecting rod, ecm file, injectors etc

Variables are-
Stroke and piston pin height

The file is written for a 7.2 inch stroke, I am using it on a 6.75 inch stroke

My question is does the timing change by using it on a shorter stroke. My initial thoughts are no As it’s off of the
crank and camshaft and both engines rotate 360 degrees.  But I’ve been told it advances timing and I’ve been told it retards timing due to shorter stroke.. I’m having trouble wrapping my head around it.  I’d think piston speeds may be different as one is covering longer distance in single revolution. but if someone can shed some intelligent light on this I’d be grateful. Thanks in advance Rob

[TJ]

I'll take an uneducated crack at it...

My guess is timing would stay the same and fuel volume would need to be adjusted... less cubes needs less fuel.

[jayb]

With the longer stroke, the piston will be down further in the bore at any given crankshaft angle.  I don't know if that would change the timing requirements or not.  If you want the piston in a specific position when the plug is fired, then with a shorter stroke you would want to advance the timing just a little to have the piston in the same location as the longer stroke would be.  (I don't know anything about diesels, but don't they use a glow plug, not a spark plug?  Do you fire a glow plug?  I thought they were powered all the time.  Clueless on that one...)

Regarding air/fuel, if the engine is equipped with an oxygen sensor with a feedback loop, no adjustment in fuel supply should be required; the O2 sensor should tell the ECU if the engine is running rich or lean, and the ECU should adjust from there.  On the other hand, if the O2 sensor is not in a feedback look to the ECU (injection system running open loop), then an adjustment in the fuel map would be required.

[FrozenMerc]

Rod length is going to play into your piston location as a product of crank angle equation as well, see an example diagram below.  Longer rods increase dwell time at top dead center.  I would suggest you put a quick Excel spreadsheet together with your variables to understand the order of events, the equation describing piston position is in the 2nd diagram, solve for x.  Injection timing is critical on a diesel with regards to piston location, the spray pattern from the injector is matched to the shape of the bowl in the piston in order to get ideal swirl and fuel atomization.  Firing that injector too soon or too late, can result in burnt pistons, poor combustion, detonation, etc.  A spreadsheet will tell you how much the piston location is going to change from one stroke length to the next with regards to crank angle. 

[ROBSREDFORD]

Rod length is not changing  both strokes use 10.660 rod.  Piston pin height varies in piston body

[BigBlueIron]

So you "made a C17" a c15 crank with a C18 cylinder packs. And running a C18 file?

TDC remains the same but piston position changes, basically in approaching tdc the piston gets there faster as it has to cover more distance in the same amount of time.

A diesel is a compression ignition engine, squirt the fuel in and it is compressed to a point enough heat is created for self ignition. (why they knock).. Glow plugs are only for getting the process started. And in some cases emission reasons. This family of Cat engine uses no such starting aid. Nor does it rely on any type of 02 sensor, and I feel I should mention no throttle plate. The intake tract is wide open path from the turbo through the cooler straight to the intake valve.. A diesel is whats considered a lean burn engine.

Engine rpm, power and behavior is directly controlled by how much and when fuel is put in the cylinder. Which with this particular engine is only controlled by "throttle" pedal input, map sensor, coolant and maybe oil temp, can't remember.

Unfortunately I can't make any tuning recommendations. But any part changes directly effect the combustion in a diesel engine. In some cases its not enough to matter. Or it can run fine for couple hundred thousand miles before the problem shows up, usually a scored liner or cracked piston.  You should see some of the endless pump settings for old mechanical engines where only one variable is different.

[TJ]

Any idea how to know if the timing is optimal?  My guess is fine tune on a dyno or measure emissions.

I’d be a little surprised if a timing adjustment is needed for  0.45” stroke change.  Since it’s compression ignition, I’m wondering if changing the compression ratio would require a timing change.

How many times do the injectors on this engine fire per compression stroke?

I’ve never played with these variables.  I just change stock parts when something wears out whether it be electronic or mechanical injection. 

[WConley]

Since the rod length is the same, piston timing will be identical to the stock engine.  HOWEVER, you now have a lower compression ratio.  This will have a strong effect on combustion chamber temperatures at the point that fuel is injected.  The engine should still run fine, but emissions will almost certainly change.  I'm thinking slightly lower NOx but probably higher particulates.  Of course you'll make less torque and have lower fuel economy, since efficiency is directly tied to compression ratio.

There is probably a way to tweak the injection curve, but that will be a ton of work!

[pbf777]

Variables are-
Stroke and piston pin height

HOWEVER, you now have a lower compression ratio.

    The compression ratio doesn't have to be different unless the specific substitutions (O.E. pistons I assume?) don't provide for equality.   

    The O.P.'s inquiry is interesting, but probably needs better definition of context in order to answer, and probably few from this vantage point could probably answer the question assuredly anyway, as this would for a truly accurate response require one (or more) of the engineers at Caterpillar familiar with this engine series to participate.   

     But, (now for the truly unknowledgeable response!  ) if the compression ratio remains similar, with not a great piston dome configuration difference; with the same cylinder heads, with port flows and combustion chamber configurations the same; valve motion events consistent; the rod length is the same but the stroke is shorter so the rod to stroke ratio does change, but common, not that greatly; and with the shorter stroke (7.2" vs 6.75") which equals approximately 94% of what it was; and realizing that for the most part as another pointed out that diesels generally present un-throttled cylinder volumes in excess of the fuel supply; why wouldn't it work? 

     Well, yes technically, the engineers would find fault; the guys over at the E.P.A. would toss a fit, and yes some tailoring probably could make it better; but I'd bet it'd run, and as being one the things one does in 'hopping-up' a diesel is to 'turn-up' the fuel (which is sorta what would be happening here as the fuel delivery would remain constant but the capacity went down) depending on where CAT left off it might not take such a hit, performance wise, with the stroke loss, at least perhaps from a seat of the pants impression anyway?   

     But, then these are thoughts from the performance oriented perspective, where we all think we can make it run better than the engineers could anyway!   

     Scott.

[WConley]

Variables are-
Stroke and piston pin height

HOWEVER, you now have a lower compression ratio.

    The compression ratio doesn't have to be different unless the specific substitutions (O.E. pistons I assume?) don't provide for equality.   

Yup - You're right Scott.  I mistakenly assumed the same piston crown / bowl construction.  If you're ending up with the same C/R you should run about the same, with a bit less torque.    Emissions - who knows??

[My427stang]

My gut response, as a diesel, it will be slightly overfueled and will like it.  I think timing is in the ball park, but quantity will be a bit high and spool quicker.

Just a WAG though