Lugging diesels = bad or newfangled myth?

[To20Chris]

The long stroke comment got me to thinking - old British engines were always small bore, long stroke. Why? Because the tax code only looked at bore! Long stroke is a mixed bag. For a given displacement it has more torque when the crank is a 90deg, but the short stroke/big bore engine has more torque when the piston is near TDC, because the piston is a pneumatic piston with more surface area. Long stroke engines have greater piston speed and more wear, made worse by the greater angle of the conrod pushing into the cylinder wall, unless you use a really long conrod, and then you get a really tall block. If you run a long stroke engine at high RPM, the reciprocating forces can get quite large, which equals more stress.

In the old days metalurgy, stress analysis, machining tolerances, lubricants, and fuels were not always as good, so low RPM was all that was practical. Given that, long stroke was no penalty and probably gives better torque throughout the crankshaft rotation. These days, something around "square" (bore=stroke) seems to work best, and that is how my Kioti, and I think most others, is designed.

 

[fishman]

Cowboydoc,

Interesting what you say about the idling diesel trucks. I guess I want some clarification. Lets say I have a Ford Superduty and stop to get a bite to eat. I leave it running for 45 minutes or so at idle. Is that bad? You mention idle controllers. Do passenger trucks have that or just semi's? What happens to a turbo during extended idle?

The reason I'm asking is that recently quite a few idling diesel trucks have been cropping up in Texas parking lots. I just want to know if it's a good thing, or (as I suspect), just something folks around here think is cool. /forums/images/graemlins/smile.gif

Oh, and I want to get one some day.

 

[Bird]

John, Cowboydoc likely knows more than I and can give a better answer, but I thought I'd mention a couple of things you might be interested in. You know my brother used to run a Matco Tool truck in the Waco area. Those tool trucks are rolling air-conditioned showrooms. They're started in the morning and not shut down again until the end of the day, and they sit for awhile at each garage they call on. My brother started with a GMC cabover, Isuzu 4-cylinder diesel, then later moved up to a GMC TopKick with a 200hp Cat turbo diesel (I'm not sure of the exact hp). Now he didn't always use it, but both of those trucks had a knob under the edge of the dash that you could pull out and turn to lock that simply accelerated the engine to a very fast idle, and you were supposed to do that if the truck was going to be left idling very long. After he got out of that business, the guy who replaced him had a Freightliner with a 300hp Cat turbo diesel, but my brother ran the route for him a few times, and I drove that truck and route for a week once. That truck had what Cowboydoc was talking about, I think; computer controlled idle. It would be sitting there idling at a "normal" or slow idle, suddenly jump to a fast idle for awhile, then drop back to the slow idle, etc. I don't know whether the Ford pickups have that or not.

I can remember in my younger days, with gasoline powered vehicles, you worried about them overheating if you left them idling too long. With diesels, they tell me you don't worry about that, you worry about them running too cool at a slow idle.

 

[Bob_Skurka]

Bird, I think your story sums it up pretty well.

On my truck fleet the older trucks don't have the computer control to alter the idle, the newer trucks do, so you can leave a newer truck at idle for a long period of time, but the computer changes the engine speed if there is no demand placed on the engine by a compressor of some other load. My trucks are considered Medium Duty and are in the Class 4 and 5 range. We don't operate the really big rigs.

You do not want a diesel running at low speed for a long time. Nor do you want to put excessive load on a diesel that is running at low rpms. Nor do you want to put a load on a diesel that will pull the engine down so low that it starts to choke itself and can't recover.

 

[Panache]

Ford Pickups since at least '99 have available as an option an AIC. Auxiliary Idle Controller. It is a box that sits on the hump. When I put on the emergency brake in park the engine idles up to 1200rpm. Cools real good in the summer. It can also be set to monitor voltage or come on automatically with a PTO i.e. hydraulic pump for the dump bed.

 

[cowboydoc]

Why would you even consider leaving it idle for 45 minutes? The only time to let one idle is like Bird said where it is constant stop and go. The idle time is usually 5 minutes or less. If it's going to be five minutes or longer you should shut it off. Most all of the newer trucks have AIC for them like Bird and Bob said.

You don't want them to idle for a couple reasons. The main reason is that it is very hard on the injector system. The injector system is no longer just the $50 injector like the old systems. You're talking $400 and $500 a piece for injectors. The fuel wash when idling for a long period significantly cuts down on their life. The other thing it does is scores the cylinder walls. When idling at low speeds the oil pressure drops. You also get fuel not getting burned that creates havoc with the exhaust and further degrades the oil. If you do idle alot it's the same service as if you were doing severe duty with the truck.

 

[Bird]

</font><font color="blue" class="small">( If you do idle alot it's the same service as if you were doing severe duty with the truck.)</font>

Yep, and another thing I didn't mention is that those tool trucks have an hour meter that's used for the service intervals instead of the odometer.

 

[Mark4510]

Gents,
I think I have the answer for this but perhaps one of you experts can line me out here.

Rarely when working around do I bog down. However, when I am dragging the arena I tend to keep the throttle at the minimum as I do when I have the box scraper on and am dragging the driveway. If I step it up, the revs jump with no real effort but I sure notice an increase in exhaust when working at min throttle. So....should I drop it down a gear and increase revs or am I okay. I read the manual and thought I was ok but wonder if I might be just a bit better off by increasing revs...

Thanks,
Mark

 

[RichNJKubota]

Use synthetic diesel engine oil so the connecting rod bearings (and crankshaft) don't wear as fast. This is where the wear occurs. I've seen it many times over the years on gas and diesel engines.
Change the oil and filter as per the owners manual and your engine should last a long, long time.

Rich

 

[cowboydoc]

I always drop a gear Mark and rev 'em up.