Is it bad to run a small engine unloaded?

[nobb]

I am a junior engineer and part of my job involves designing process plants which use large, industrial engines from Caterpillar and Waukesha. I noticed that the manual for some of these industrial engines warn to not operate for long periods under low load (under 50% rated bhp) as it will be detrimental to service life.

So out of curiosity...on a consumer level with automobile and small engines, is it harmful to idle or rev an unloaded engine? If so, what are the specific reasons?

 

[OneEyeJack]

Running "unloaded" might be a problem because of lack of heat in the system. Engines are designed to work at operating temperatures and anything less is a compromise. Everything from the fuel system to the cooling and lubrication systems can be effected by running at "idle" speed. Fuel might not get atomized, additives in oil might not be effective and in some engines vibration might be an issue. Deposits can built up in the cylinders and manifolds and the oil can become contaminated.

What's best for an engine is to start it when you need it, run it long enough to get it up to temp and run it for as long as needed, then shut it down.

 

[4wheeldog]

Cylinder pressure is the primary force keeping piston rings in close proximity to the cylinder walls. Engines run excessively under low loads can tend toward increased blowby, and excessive deposits on the pistons (Ring lands as well as under and over the piston top) and in the cylinders.

That along with the low running temps are reason enough to avoid idling, or slow warmups. Running near rated load is what they are designed to do.

This is my excuse to regularly wring out every engine I own.

 

[440Magnum]

Originally Posted By: 4wheeldog
Cylinder pressure is the primary force keeping piston rings in close proximity to the cylinder walls.


And I would add, getting the RPM up for at least part of the duty cycle is also required to let the rings move correctly within the ring gaps on the pistons, preventing "stuck rings." Constant low-RPM operation keeps the rings pushed down on the pistons due to combustion chamber pressure being the dominant force, higher RPM lets them lift up and contact the land above the ring gap.

 

[CT8]

Yeah and no and who cares. I worked fleet maintenance for most of my life and properly maintained and not abused the engines last a long time. Although to much low load low rpms will increase carbon deposits and some what fuel dilution of oil and a few other things.

 

[ronbo]

COP car engines do a lot of idling, it doesn't seem to effect there engine life. They never seem to turn them off, since they need power for radio, laptop, etc.
The Police engines are built more rugged than those in regular passenger cars.

 

[JHZR2]

Large diesel engines have wet stacking issues if run long-term at light loads. It is an issue in lots of applications.

I suspect that for small gasoline engines, running high RPM and high temperature, that other than some fuel dilution, you don't have much of a concern.

There have been reports of automobile engines that are lightly used having issues when they encounter high loads or other conditions, but those are the exception, IMO... And moe related to oil fuel and water diluton and other such things...

 

[CT8]

Originally Posted By: ronbo
COP car engines do a lot of idling, it doesn't seem to effect there engine life. They never seem to turn them off, since they need power for radio, laptop, etc.
The Police engines are built more rugged than those in regular passenger cars.
They are not.

 

[4wheeldog]

Originally Posted By: ronbo
COP car engines do a lot of idling, it doesn't seem to effect there engine life. They never seem to turn them off, since they need power for radio, laptop, etc.
The Police engines are built more rugged than those in regular passenger cars.


Taxis would be a far better example.

 

[nobb]

Interesting thoughts! By running unloaded, I mean literally nothing connected to the shaft. A car engine idling would still be loaded by the alternator, water pump, fan, etc.

 

[Rand]

to OP:
yes, for example you wouldnt want to run your car engine at 6000rpm in park or neutral for extended periods

many other small engines such as lawnmower have no such thing as low load and are limited to a certain RPM that is below the engines design but the max for the blade tip speed.

 

[Merkava_4]

Originally Posted By: nobb
Interesting thoughts! By running unloaded, I mean literally nothing connected to the shaft.


You mean like a push mower with no blade attached?

 

[SwampSurvivor]

Originally Posted By: 4wheeldog
Originally Posted By: ronbo
COP car engines do a lot of idling, it doesn't seem to effect there engine life. They never seem to turn them off, since they need power for radio, laptop, etc.
The Police engines are built more rugged than those in regular passenger cars.


Taxis would be a far better example.


Taxi would be a worse example.

A cop car is idling most of the day doing nothing.

A taxi is run around the clock 100% of the time, always at temperature and always running at speed with a load.

 

[4wheeldog]

Originally Posted By: Miller88
Originally Posted By: 4wheeldog
Originally Posted By: ronbo
COP car engines do a lot of idling, it doesn't seem to effect there engine life. They never seem to turn them off, since they need power for radio, laptop, etc.
The Police engines are built more rugged than those in regular passenger cars.


Taxis would be a far better example.


Taxi would be a worse example.

A cop car is idling most of the day doing nothing.

A taxi is run around the clock 100% of the time, always at temperature and always running at speed with a load.


Obviously, you have never driven a taxi.

 

[Eddie]

If you remove the blade from a small mower engine you are subjecting the crankshaft and connecting rod to much more cycling impacts. Whether is enough to damage anything, I don't know. It is one reason car engines use harmonic balancers on the end of the crankshaft opposite the flywheel. ed

 

[95busa]

Originally Posted By: Miller88
Originally Posted By: 4wheeldog
Originally Posted By: ronbo
COP car engines do a lot of idling, it doesn't seem to effect there engine life. They never seem to turn them off, since they need power for radio, laptop, etc.
The Police engines are built more rugged than those in regular passenger cars.


Taxis would be a far better example.


Taxi would be a worse example.

A cop car is idling most of the day doing nothing.

A taxi is run around the clock 100% of the time, always at temperature and always running at speed with a load.

Police car engines are built no more or less rugged than their civilian equivalents. Usually an already rugged engine is selected, like the 2V 4.6, which is the best police car engine I have ever seen. Given good maintenance, it is nearly indestructible. The differences are in the cooling. Typically they use a larger capacity radiator, an engine oil cooler (lately oil to water type). Other differences power steering cooler, transmission cooler, bigger brakes, better pads, different suspension tuning (springs, shocks, swaybars), calibrated speedometers, deeper gears, no sound insulation, stripped interiors, big alternators, no resonators in the exhausts, speed rated wheels and tires, etc. Differences vary model to model, but thats about the size of it. I have been assigned old Caprices, Crown Vics, Chargers and Impalas.

 

[JTK]

Originally Posted By: nobb
I am a junior engineer and part of my job involves designing process plants which use large, industrial engines from Caterpillar and Waukesha.



Sounds COOL!

Not really engine related, but I know in my line of work, I've been around some giant Sulzer-Burckhardt recip compressors (in O2 service) that sound like absolute p00p until they build-up some pressure to keep the pistons centered.

 

[eljefino]

Something about con-rod bearings don't like low load. Like they're expecting to be compressing air + fuel during one stroke and if they aren't they overshoot themselves and stretch themselves out. Wish I could find my reference.

 

[Cujet]

Interestingly, industrial engines based on automotive engines really do last. In fact, at the airport, they outlast car engines when measured by hours or years.

I'm not sure the "Low Loads" on automotive engines is the cause of shorter life, but is interesting to consider.

It's not unusual for a Ford 6 cylinder aircraft GPU (ground power unit) generator engine to accumulate 25,000 hours. A fair comparison might be 1,250,000 miles at 50MPH average.

 

[ronbo]

Originally Posted By: 95busa
Originally Posted By: Miller88
Originally Posted By: 4wheeldog
Originally Posted By: ronbo
COP car engines do a lot of idling, it doesn't seem to effect there engine life. They never seem to turn them off, since they need power for radio, laptop, etc.
The Police engines are built more rugged than those in regular passenger cars.


Taxis would be a far better example.


Taxi would be a worse example.

A cop car is idling most of the day doing nothing.

A taxi is run around the clock 100% of the time, always at temperature and always running at speed with a load.

Police car engines are built no more or less rugged than their civilian equivalents. Usually an already rugged engine is selected, like the 2V 4.6, which is the best police car engine I have ever seen. Given good maintenance, it is nearly indestructible. The differences are in the cooling. Typically they use a larger capacity radiator, an engine oil cooler (lately oil to water type). Other differences power steering cooler, transmission cooler, bigger brakes, better pads, different suspension tuning (springs, shocks, swaybars), calibrated speedometers, deeper gears, no sound insulation, stripped interiors, big alternators, no resonators in the exhausts, speed rated wheels and tires, etc. Differences vary model to model, but thats about the size of it. I have been assigned old Caprices, Crown Vics, Chargers and Impalas.


Maybe in the present COP engines are the same BUT I believe in the past they were different. For example the below.

"The Crown Victoria Police Interceptor came equipped with many heavy duty parts such as a revised transmission, and a 187 kW (254 PS; 251 hp) engine. link: http://en.wikipedia.org/wiki/Ford_Crown_Victoria_Police_Interceptor

2004–2011 Ford Crown Victoria Police Interceptor, 250 hp (186 kW) and 297 lb·ft (403 N·m)

"The 4.6 L 2V has been built at both Romeo Engine Plant and Windsor Engine Plant, and the plants have different designs for main bearings, heads (cam caps: interconnected cam "cages" vs individual caps per cam journal), camshaft gears (bolt-on vs. press-on), valve covers (11 bolts vs. 13 bolts), crankshaft (6 bolts vs. 8 bolts), and cross bolt fasteners for main bearing caps" link: http://en.wikipedia.org/wiki/Ford_Modular_engine