Diesel Talk
Q: I am a retired diesel engine design engineer and cannot overlook the technical mistakes in the article on diesel engines (“Diesel Talk”) on page 60 of the September 2006 issue of Family Motor Coaching magazine. The author refers to “piston movement … allowing a fresh charge of the fuel/air
mixture …” This is in error, as the only thing that enters the cylinder during the inlet stroke is filtered air and does not include any fuel. The author is confused with a spark-ignition engine where there is indeed a fuel/air mixture. This erroneous statement is repeated in a later paragraph as the mixture does not explode (as there is no mixture), but the ignition is caused by the injection of fuel into the hot compressed air.
I hope this correction is published, as a lot of novices are going to be thoroughly confused.
Robert J. Dillon, F309323
Gainesville, Florida
A: The above letter is representative of a number of letters we received in response to my error in the “Diesel Talk” article that appeared in the September issue. This is a correction of that article.
As stated in Mr. Dillon’s letter, I inadvertently confused the cylinder charging sequence of a diesel engine with a four-stroke gasoline engine. I was trying to describe the sequence in very basic terms and I thought I was explaining how a pre-turbocharged diesel engine operated. I’m sorry for the confusion I caused with my use of the term “fuel/air mixture.” Actually, a modern electronically controlled turbodiesel engine operates in the following manner:
The piston is pulled downward by the rotation of the crankshaft, which creates a near vacuum in the cylinder; the intake valve opens.
The turbocharger shoots in its charge of air at a much higher pressure than the ambient air pressure (this is why turbodiesel engines are much less affected by high altitude than normally aspirated engines).
As the crankshaft pushes the piston upward, the air is compressed, which causes it to become superheated.
As the piston moves upward in the sealed cylinder, the air is compressed by the piston; this quickly superheats the air to high temperatures and pressures critical for ignition the moment fuel is injected.
In a matter of microseconds, the fuel is injected in three pulses; each pulse is critically timed electronically. For want of better terms, they could be called “pre-, present-, and post-ignition.” The pre-ignition pulse is fired, then the present-ignition pulse is added to the mix, and then the post-ignition is injected just as the piston begins its downward power stroke for an added boost, much like an afterburner in a jet plane. This triple-pulse is one of the reasons modern diesel engines are much quieter than their predecessors.
When the crankshaft pushes the piston upward again, the exhaust valve opens and the exhaust gases are pushed out of the cylinder.
And then the whole sequence starts over again.
Jim Brightly, F358406
Technical Editor
Dakota Discharge Part 2
Q: In the September 2006 issue of FMC, page 20, you published a letter from Larry and Marilyn Knight (“Dakota Discharge”) about 2005, 2006, and 2007 Chrysler products (especially Jeeps) that cannot be towed without disconnecting the negative battery cable. This is incorrect.
Here is what is going on. On the older vehicles, in order to release the steering wheel (so the front wheels will track), you put in the ignition key and turned it to the first position. This released the locked steering wheel and activated certain ignition circuits. The draw was so low it wouldn’t do much, if anything, to drain the battery.
Now with the later models, there is a lot more electronic stuff that is activated when the key is in the “On” position. That’s what’s causing the problem. So what’s the answer? On the later models, the steering wheel is not locked; therefore, the ignition key does not have to be inserted, let alone turned on.
What they obviously were doing was how we all did it in the past. They’re now causing their own problems. But don’t take my word for it. If there isn’t one of these vehicles in your parking lot, go to a nearby Jeep dealer and try it for yourself. Don’t just call a salesperson, because they may not know what you are talking about or may give you an answer based on old thinking.
Allan Bluestone, F342030
Fountain Hills, Arizona
Q: My current towed vehicle is a 2006 Grand Cherokee Limited with the 5.7-liter Hemi. I have not experienced any battery failure while towing, such as described in the Knights’ letter. I suspect that Chrysler products that require the ignition key to be in the “On” position in order to unlock the steering column security lock, while towing, may have this problem.
A friend who drives a Chrysler van has a battery disconnect, obtained from NAPA, as his battery died during towing. He does have to leave the key in the ignition in the “On” or “Unlock” position.
The towing instructions for my Grand Cherokee tell owners to only use the ignition key, placed in the “On” position, to verify that the electronic transfer case is indeed in neutral before towing. After verification, the key is turned off and removed, as there is no steering column lock. In other words, nothing that requires electrical power is left on while towing. I have the Blue Ox wiring harness, which goes to an additional light in the car’s taillight assembly. The coach, therefore, provides the power to operate the Grand Cherokee’s turn signals, stoplights, and taillights, not the towed vehicle.
Hope this clears up the mystery.
Art Gray, F223762
Livingston, Texas
Q: I don’t think Larry Knight has his facts straight on this issue, at least as far as the Jeep Grand Cherokee is concerned. I suggest that he left his lights on. How can a battery go dead when the transmission is in park (transfer case in neutral) and the key is off and removed? Power to the taillights is supplied via the pigtail from the host vehicle. Two large Jeep dealers concur. We have been towing a 2006 model with no problems, as have many people I have talked to in RV parks since reading your column. I think a correction is in order since the column has caused a lot of folks concern.
Jack Wright, F255665
Indio, California
A: We checked with a representative from Daimler-Chrysler about this issue and were told that the steering column on current Jeep Grand Cherokee and Commander models does not lock when the key is removed. The rep did say that trying to move the steering wheel with the engine off may be difficult and give the impression that the column is locked if not checked closely.