Diesel engine testing

Surely this all depends if the engine was on a dyno or actually running through a gearbox. without a GB attached a diesel engine on the bench will run any revs you want it to. Most modern day diesels will bench at well over 7.5k rpm. Its not a case of diesel not being able to be burned quick enough as long as its getting the air it will burn all day long, that and the fact that the pressure from common rail systems is enough to blow the skin off your finger down to the bone shows its more than capable at fueling.
 
Butters said:
that and the fact that the pressure from common rail systems is enough to blow the skin off your finger down to the bone shows its more than capable at fueling.
Click to expand...

Not really when you factor in its burn time, to fuel for the next stroke you
need be injecting right in the current stroke combustion phase.

A dyno will load up exactly like a gearbox if you want it to.

Im of the opinion this will be a powered engine with a electric motor.
 
Butters said:
Surely this all depends if the engine was on a dyno or actually running through a gearbox. without a GB attached a diesel engine on the bench will run any revs you want it to. Most modern day diesels will bench at well over 7.5k rpm. Its not a case of diesel not being able to be burned quick enough as long as its getting the air it will burn all day long, that and the fact that the pressure from common rail systems is enough to blow the skin off your finger down to the bone shows its more than capable at fueling.
Click to expand...

Why is burn time not relevant, and yet you are saying the pressure in a common rail system is?

"Any revs you want"? So you don't feel that it could possibly be limited in any way by the valve gear or strength of the bottom end? If it was a production engine it would be limited by the ECU long before 7500RPM anyway.
 
It's pretty obvious diesel will not burn quickly enough with 7,500rpm cycles.

Hence it'll be making virtually no power if it was gobling air and drinking fuel at those revs. If it's making no power that's pretty pointless as a stress test, which leads me to believe this was a durability test with the engine just being spun up to 7,500rpm rather than running at 7,500rpm.

Stress test you'd do with the engine under load and producing power i.e 4,000rpm. Or simply throw stupid amounts of boost at it, or both!
 
Durability testing is about seeing how long the engine last and monitoring parameters.

Hardly about going well past the design spec of an engine.

Diesel engne speeds are limited by the combustion time of diesel aswell as the heavy parts and I'd be pretty sure the valvetrain would struggle at 2500rpm over the max engine speed.

Hard to comment without context though. My durability test is 800hrs at full load 4000rpm on a 'normal' 2.0D engine.
 
Simon said:
Durability testing is about seeing how long the engine last and monitoring parameters.

Hardly about going well past the design spec of an engine.

Diesel engne speeds are limited by the combustion time of diesel aswell as the heavy parts and I'd be pretty sure the valvetrain would struggle at 2500rpm over the max engine speed.

Hard to comment without context though. My durability test is 800hrs at full load 4000rpm on a 'normal' 2.0D engine.
Click to expand...

you owe me a pro evo game Si, after spouting out your lies about beating me! ;)
 
Simon said:
I'd be pretty sure the valvetrain would struggle at 2500rpm over the max engine speed.
Click to expand...

Maybe that was the whole point of doing 7,500rpm?

To purposely see how long the top end could cling on to dear life.
 
merlin said:
Maybe that was the whole point of doing 7,500rpm?

To purposely see how long the top end could cling on to dear life.
Click to expand...

Probably, It's likely it was motored in that case, the only way you'll do that in a car is by downshifting at the redline, don't know how many people could hold a downshift for 1200hrs though :p
 
Cool. :)

Do you agree the engine would be producing negligible amounts of power @ 7,500rpm if it was actually "running"?
 
If it was a normal engine as described then i doubt there would even be any fuel up there. The engine would purely be acting as an air pump.

Just asked my boss, he can't understand why they would run so high. 500rpm over max is the industry norm.
 
Last edited:
If a conrod failed it would be a perfectly good reason for a fatigue test.

1200 hrs at 7500 equivalent to i dunno 4000 hrs at normal rpm limit but the test is complete way faster.

Conrod is exposed to tension stress on each revolution, the higher revs means the piston speed is higher and there for the acceleration forces on a reciprocating piston is higher. F=ma means you get a higher force, for the same cross sectional area a higher force = a higher stress.

From the stress values you can predict the number of cycles to failure. If that is 1250hrs and you expect the test to be atleast 1200 (540 million cycles) then failing at 1140 means you havent quite understood the loads properly = not good. A running engine rather than a driven non burning engine will generally see higher tensile forces in the conrod as theres nothing to compress. Similar with valves assuming the valvetrain can prevent the bounce.
 
Jonnycoupe said:
If a conrod failed it would be a perfectly good reason for a fatigue test.

1200 hrs at 7500 equivalent to i dunno 4000 hrs at normal rpm limit but the test is complete way faster.
Click to expand...

Its exponential though. 7500rpm would have massive forces that would be unrealistic of the real world.

Even a full rpm full load test for 800hrs is unrealistic in the real world.
 
Exactly on a logerithmiic scale a typical SN curve is the time to failure is MUCH shorter when you up the stress. Hence its a perfect method to employ on a real world test where product lead times are desired to be as short as possible

Its unrealistically high but on a derv safetly margin aslong as you dont exceed platic yield the prediction should be pretty accurate for any sort of product proving. It just clarifies and confirms any CAE.FEA work. No doubt failure pre-1200 hrs has raising questions over the data fed into any modelling software.
 
You can only create that SN curve with data from previous testing though

This isn't a tensile test but a very complex piece of machinary.
 
You have SN curves for any alloy you want, the whole point of them is that you dont have to test each component in any assembly as long as you understand whats happening at the extremes of the loading. Rag it til it breaks, aslong as its inside the design envelope it would pass it as fit for purpose its safe. You can do the same for all the parts, as i hinted at with the shear of a gudgeon. Factoring up accelerating forces with rpm is the only reason i can think of for inducing such stresses if the engine isnt self powered. Same thing as F1 but the revs if so high and lifing so short they can just run them at real time to form life limits, not quite as practical as a 100k road car worthiness testing.

Just suggestions as to why the revs would be so high :p
 
Last edited:
Theoretically a diesel engine could rev as high as a gasser.

The problem is that your injection window gets shorter as the engine speed increases.

A modified TDI can do 5200rpm but that's the limit due to fueling and ECU protection.

I don't think even tractor pullers can run as high as 7500 with the most i've witnessed at (12v71) at 6500rpm.

to summarise then we know the engine was turned at 7500rpm for durability and reliability. Although why it was this high is in my mind odd.

Perhaps they are testing Piezo Injection technology
 
Last edited:
Butters said:
The problem is that your injection window gets shorter as the engine speed increases.

Perhaps they are testing Piezo Injection technology
Click to expand...
I'm sure I read that the piezo injectors in the VAG diesels use five separate fuel injections per cycle. In a four pot at 4500rpm that'd be 750 fuel injections per second if my maths is right :eek:
 
You must log in or register to reply here.
Back
Top Bottom