No-load engine testing

[WillGysi]

Hi everyone,

I'm a long-time reader of this forum (as it pops up in google whenever I search a question) but have one now that I haven't been able to directly find the answer I am looking for.

I am currently in a bit of an argument with a friend. We are building an engine with some design changes for our senior design project (mechanical engineering seniors) and will be testing our engine in comparison to the test results of a baseline engine. (ie: test engine, make changes, re-test).

I am trying to convince that we need to have a dyno of some sort to load the engine. He thinks that a no-load condition will be satisfactory.

We want to find our efficiency so determining HP,Torque, rpm, bsfc, bMEP, etc is ideal.

He wants to run the engine with no load and just see how long it takes to burn through a set amount of fuel.

What do you guys think here?

 

[140Airpower]

Will, how does your friend propose to measure HP, Torque, rpm, bsfc, bMEP, if not with the engine under load? I suppose that with proper instrumentation you can do an acceleration test. In that case the engine is under the load of accelerating against its rotational inertia. You could get fast readings of instantaneous torque, the calculated hp, fuel injector pulse width for a fuel consumption calculation, etc. However, you will not likely get optimum figures, just whatever they happen to be. Of course that may be sufficient for the assignment.

 

[WillGysi]

That's the thing, he has no solution for determining the majority of that. RPM you can get through a photo-diode or something similar. All he thinks we need to do is give it a set amount of fuel and wait for it to stop running. I said that's a bit too back of the envelope style for what this project needs.

 

[moon161]

Depends completely on your changes. If you put a resonator on the intake and you never put the motor in that RPM band, you'll never use the resonator, will you?

I think your project advisor would be the best one to set the bar on this.

 

[dicer]

Your in luck. Since it does take a certain amount of power to run the engine. So you need to figure out how much that is.

 

[dgallup]

with no load you have no brake horsepower, no torque, infinite BSFC (divide by zero), etc. I fail to see what value this has.

----------------------------------------

The Help for this program was created in Windows Help format, which depends on a feature that isn't included in this version of Windows.

 

[patprimmer]

To get meaningful results you need a decent load of some sort.

It can be to drive a flywheel or lift a load via a winch or drive a generator or drive a water pump or drive a vehicle along a specific road in both directions for accuracy.

The load needs to be high enough to test at full throttle for a reasonable time without over speed of the engine. It preferably needs to be variable so you can hold the rpm stable at specified rpm, but if you can have the engine reacting against a load cell while accelerating and you can data log both load and rpm, you effectively have a dyno.

Regards
Pat
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[1gibson]

If that information were in any way related to the project, you would reduce the idle RPM as much as possible and modify the engine to keep it from dying at low rpm. Seems very unlikely that is the intent of the exercise.

Maybe start by telling your friend to read the instructions?

 

[ivymike]

as 1gibson said... there are ways to win the "runs a very long time at no load" game, but few of them will have practical value for an engine.

Note that the more-efficient cars' engines on the market today SHUT DOWN instead of idling at no load... at no load you need no engine!

 

[WillGysi]

Thanks for the replies everyone.

For those of you considering the lower rpm loophole to this, the engines would be tested and compared at the same rpm.

 

[dicer]

I don't know how else to qoute this statement, I don't see a quote button any place on this deal.
So

""""with no load you have no brake horsepower, no torque, infinite BSFC (divide by zero), etc. I fail to see what value this has. """""
WHAT? Yeah if its turned off, ie not running. But when it starts there is torque, there is rpm, there is BSFC for the simple fact that the gas starts to disapear. To keep the engine running there is a certain amount of power or torque needed to accomplish that. Agreed its not alot of loading but still it does exist.

 

[Lou Scannon]

BSFC is Brake specific fuel consumption. This requires work to be done & measured, external to the engine, in order to have a non-zero value in the denominator.

"Schiefgehen will, was schiefgehen kann" - das Murphygesetz

 

[SomptingGuy]

Why not just cable-tie the throttle open and measure the time it lasts? That's quite often done at bike rallies I've been to. Extra points for method of expiration.

- Steve

 

[ivymike]

when it starts there is torque, there is rpm, there is BSFC for the simple fact that the gas starts to disapear. To keep the engine running there is a certain amount of power or torque needed to accomplish that. Agreed its not alot of loading but still it does exist.

http://en.wikipedia.org/wiki/Mean_effective_pressure

Brake mean effective pressure (BMEP) - Mean effective pressure calculated from brake power
Indicated mean effective pressure (IMEP) - Mean effective pressure calculated from in cylinder pressure, average in cylinder pressure over engine cycle, 720°.
Friction mean effective pressure (FMEP) - Theoretical mean effective pressure required to over come engine friction, can be thought of as mean effective pressure lost due to friction. BMEP = IMEP - FMEP

BSFC is calculated based on BRAKE power, which is what you have left over after you do all the things to keep the bare engine running. At idle, brake power is zero. Yes, the engine still burns fuel.. but the BSFC is indeterminate.

 

[1gibson]

"For those of you considering the lower rpm loophole to this, the engines would be tested and compared at the same rpm"

Well if that RPM is the idle speed of the baseline engine, then your friend is spot on, you don't need to know anything else. I doubt that's the case.

Next strategy: use as much EGR as possible, let the engine operate at highest safe temperature, and lean it out.

As a side note, you should know by now that there is no such thing as a loophole. Rules are rules, and "not rules" are not rules.

 

[turbomotor]

Hi WillGysi,

If you will excuse the sarcasm, finding the thermal efficiency at the "no load" condition will be both easy and accurate, as it will be (by definition) zero.

In an attempt to be just slightly more helpful: How many cylinders does your engine have? What is the modification that you plan to make? Is qualitative comparison data sufficient to prove your case? (i.e. is this better than that?)

You can make a cheap "load" dyno by running your multi-cylinder engine as a single cylinder test engine with the remaining "unused" cylinders used as a friction load device. The manifold pressure required to achieve a given rpm will increase if you only fire a single cylinder, indicating an increased friction load (on a per fired cylinder basis).

Or, if you want to get a bit fancier, take the spark plugs out of the unused cylinders and replace with a pipe that includes a flow check valve (one-way reed valve or similar). You will get a low cost "jake brake" effect, as the fiction load required to drive the unfired cylinders will go up. Make sure you also defeat the fuel flow to the unfired cylinders, or you make have some nasty surprises.

I hope this was a helpful consideration for single fired cylinder comparative performance testing done on a shoe-string budget.

Dick

 

[moon161]

Bottom line, for me a test simulates operating conditions, or relevant aspects thereof. If your goal is to reduce idle fuel consumption and emissions, that's a good thing. There are millions of people stuck in big city traffic worldwide that would agree. However if you assert that you have fuel savings while producing useful shaft work, you better load the motor and spin it faster to prove it.

If you were in industry and your boss was ready to commit millions of dollars to engine modifications, I expect they would dyno the bejesus out of it first. I've worked for places that ate it big because they didn't test new designs.

 

[LionelHutz]

As Dick already pointed out, you don't need to do any tests to find the efficiency without a load. It is 0.

You will need a load on the engine if you want to test/prove anything remotely useful. I will have to disagree with Pat. It can't be an inertia load (ie flywheel). You need a steady load you can apply long enough to do useful measurements.

 

[patprimmer]

Inertia dynos are pretty common now. With modern data logging they can provide useful info. Of course I still much prefer a brake type dyno and steady readings.

Regards
Pat
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[LionelHutz]

Yes, but measuring BSFC or efficiency at fixed operating rpm's is rather difficult with an inertia load.