The below excerpt was taken from http://www.tejascoach.com/index.html, which I mentioned in a different post. According to this information, I have not been driving correctly. I was under the impression that when climbing a hill I could keep the transmission in the current gear until the engine was almost at the shift point to the next lower gear, and then downshift. The quote below seems to be saying that if you can not comfortably maintain speed in the current gear, you should already be downshifting instead to letting the engine continue to fight the losing battle. I am sure that some of you pros and DD experts will be able to confirm/deny this.
"The most common error made by both seasoned and rookie drivers that can eventually ruin your engine, cost you thousands of dollars or just make you late to your next appointment!!!
After talking to many folks over the years and repeating the following information I fell it is past due time to make this information available for easier access.
The Detroit Diesel 2 stroke family of engines are a very lasting and enduring breed. (They have been around since the 30's) There is an old saying that if you baby your engine it will last longer, except if it is a Detroit 2 Stroke, also referred to in the past as a "Screaming Jimmy". If you baby one of these engines you will end up "Lugging" it and causing it to look like the photos illustrated in the DDC Oil section of this web site. See DDC OIL Some of you will have been told the technique by Old Drivers that when driving a Jimmy you should start out the day by placing your hand on a bench and take a hammer and smash your thumb, then take your anger out on the engine because you caint(sp) hurt it by driving it hard!! Another version was as you are climbing into the cab, just slam the door on you finger, then take it out on the Jimmy.
Lugging
Lugging is when one asks the engine to go faster that is has the capability to do in the given condition that one asks it to. Such as starting to climb a mountain, or climb a hill, or just climb any slight grade and the engine will not accelerate beyond the speed you are attempting to maintain. The situation is that you are in too high of a gear for your present condition. This could be caused by trying to pull too heavy of a load, having too high of a head wind, engine getting too old to pull what it used to, clogged up fuel filters, clogged up air filter, loss of turbo boost due to leaking boost pressure, brakes dragging or park brake not released completely, flat tires, and on and on.
At engine temperatures below 186 degrees the piston is smaller at the top than it is at the bottom. At 186 degrees, the piston is the same size at the top and the bottom, this condition is altered by the crown of the piston which gets all the heat from the burning of the fuel.
Lugging creates heat in the engine which in turn will heat the oil and thin it causing oil pressure to decrease and at a point it will not have sufficient pressure to flow through the crankshaft up an oil galley inside the connecting rod into the wrist pin boss to lubricate the pin and then exit out the top of the connecting rod via the spray nozzle spraying and cooling the underside of the piston crown. At a point the lugging will cause the crown of the piston to become larger than the cylinder liner. There are times when this may not be evident via the water temperature gauges until they start to climb. If the temperature is a result of lugging, the piston crowns beat the water in the race to get hot.
With that said, there are limitations that one should know and understand where this "hard driving" does not apply! After you attain the highway speed that you are comfortable with or after you have attained the fastest speed the coach can reach, then you should back off the pressure on the accelerator pedal to a point approximately 1" off the floor board. When you do this the engine should maintain the same speed you were doing prior to releasing the pressure from the pedal. You may have to experiment with this in minor adjustments until you get the feel of where your particular vehicle's "grove" is. It may be that you can back off more than an inch or it may be less than an inch. When you drive with the pedal full on the floor you are WASTING FUEL, SMOKING UP THE ATMOSPHERE, and DESTROYING your engine. Again refer to DDC OIL. After you have perfected this technique, you can practice on doing this in every gear to learn to feel the acceleration of the engine with out flooring the pedal through out each shift. It is a constant working of the ankle bone and it why there is a distinct difference between a driver and a good driver.
Another condition where the above applies is when one is starting to climb the mountain, or climb the hill, or just climb any slight grade. On any of these conditions the proper approach is to keep your foot OFF of the FLOOR, Do NOT put the "Pedal to the Metal". The critter is designed to apply the proper amount of fuel to climb the mountain, hill or any grade AUTOMATICLY with the governor."
Here's a different rule of thumb. If your Detroit 2 stroker has either 55 or 60 injectors and is properly timed and tuned, then the engines are made for you to keep your foot into it and just drive the bus. Do you think hired bus drivers cared one bit about lugging the engine? If you have larger injectors, then watching the exhaust is another way. If you're producing black smoke, you're also creating too much heat. It is always a good idea when pulling a long grade, to be in a gear that you can have the engine revved up around 2000rpm with partial foot pedal. On my bus with the V730, I let the bus slow down to the point where I can shift into first manually then with my foot about an inch off the floor, it will bump into torque converter lockup. With the torque converter locked up, you'll eliminate a big percentage of heat buildup in the transmission.
If you have either a DDEC 2 stroker, or any electronic 4 stroker, just drive it as hard as you like. The sensors on the engine will not allow you to hurt the engine.
If you have a mechanical 4 stroke, you should have a pyrometer and be vigilant about watching the temperature and remember that exhaust above 1200 degrees cost money.
Just as a last note-all bus and truck engines are made to go up the hill with your foot on the floor (despite what the previous post says). To prove my point, Detroits Series 60 has a maximum horsepower output for trucks at 515hp @ 1800rpm. Yet for marine use, they have a 825hp at 2100rpm version. Even if you had a big enough radiator for the 825hp version, structurally the engine wouldn't take lugging, since on a boat the propeller produces its' top horsepower in the top 150rpm of the engine. Good Luck, TomC
Lin, great post, TomC, great input. Wondering if the article could have been intended for the "younger" generation because we (I) of the older crowd were generally taught the "correct" (?) way to drive our mightly Detroit Diesel 2 strokes?
Driving an oil/gasoline tanker with the "318" and fire apparatus (various models--all pumped way up) we just kept our foot buried in it and let the governor (if any) sort out everything else. They ran best at or above 2000 rpm--all the time.
When the fire shops repowered an old Crown Fire Coach (engine) with a BC400 Cummins, the driving techinque had to change. The Cummins, a fine mill, was not driven like a "Screaming Jimmy". The Big Cam was "more civilized" HB of CJ :) :) :)
I have heard that to get yourself in the right frame of mind to best drive a screamin jimmy is to slam the door on your hand ;D ;)
This post in my mind is right on the money. I took a job out of high school as a seismic vib driver. We were all young and being pushed hard for production. When the Jimmy would over heat we would go back to the engine hold the blower open and restart the engine. When the temp came down and the safety would keep blower open we would jump back in and go back to work. I'm not saying that this is how we should treat our bus engines, but the engine is built to work and is made to take it. I also fly a Cessna 182 with a continental 0470 engine. This engine is made to be ran hard. Running hard and abuse are two different things.
John
Quote from: Jriddle on November 04, 2008, 06:19:30 PM
Running hard and abuse are two different things.
John
You hit the nail on the head with that one ;) ;D