Re: [A/S] Diesel vs. Gas engine operating costs

["Oliver Filippi" <ofilippi@xxxxxxxxxxxx>]

I think the current discussion of economics of diesel engine vs. gasoline
engine operating and maintenance costs has, for the most part,  missed the
main event.

There are three classes of costs associated with a vehicle's power plant.1)
Capital investment, 2) Fuel related operating costs, and 3) Maintenance
costs (mostly related to use) to keep the engine running.  The first is a
fixed cost associated with the time period of ownership, and the other two
are variable costs and a function of use.

1. The up front, fixed cost of the engine itself.  This reflects the cost
(current interest rate) of the money tied up.  Clearly a diesel engine in
today's marketplace is more expensive than a gasoline one of comparable
quality and performance.

[However, note that auto makers do not necessarily price their products
based on manufacturing costs.  In the early eighties, or thereabouts,
Mercedes charged LESS for their diesel engine powered cars than their
gasoline engine counterparts.  The reason was that they wanted to encourage
diesel engine sales over gasoline sales to lower their corporate average
fuel economy figure (CAFE) so as not to be subject to a US imposed fine for
fuel efficiency (Mercedes gasoline engines were not gems of fuel
efficiency).  Thus they sold the ordinarily more expensive to manufacture
diesel for less than the gasoline engine.  Also, production volumes have a
significant impact on the final cost.  High production volumes can drive
down unit costs.].

For now, let's assume that the diesel costs more to purchase.  For the
diesel to be competitive on a cost basis over its useful life, it has to be
cheaper to operate. This implies cheaper fuel costs and/or maintenance
costs, sufficient to overcome the higher purchase price.

2. Fuel costs.  Modern engines are remarkable in their fuel efficiencies
compared to those of a decade ago.  This is due to improved designs, more
sophisticated computer controlled engine and fuel management systems,
improved lubricants (typically lower viscosity (thinner) oils) and friction
reducing additives.

Diesel  engines are inherently more fuel efficient than gasoline engines for
two fundamental reasons. 1) The diesel engine combustion cycle, properly
managed (and with a turbo charger), achieves essentially complete
combustion - which means that most all of the heat units (BTUs) in the fuel
are converted into power.  Also, on a volume basis, diesel fuel contains
more heat units per gallon than does gasoline.  Thus,  you need less diesel
fuel to produce the same amount of heat, and hence power.

3. Maintenance costs which includes tune-ups, filters and oil changes. Once
again, automakers have made significant improvements in both gasoline and
diesel engines to extend the oil change interval - primarily due to both
better lubricant technology and manufacturing the parts to much smaller
tolerances which permits the use of lighter weight, more slippery (lower
friction) oils.  Nevertheless, a diesel engine typically should use a more
expensive, higher performance oil for maximum longevity.

Translating the higher cost oil into a negative is really missing the point.
Remember the fundamental law of mechanical devices: OIL IS CHEAPER THAN
METAL.

In my experience I have found that most automotive dealerships (other than
large truck dealerships) DO NOT carry the best oil for maximum engine life.
Some of these lubricants are frequently not even carried by most auto parts
stores.  With my first Mercedes diesel, I actually bought a 55 gallon drum
of diesel engine rated oil and changed the oil myself.  Now, in my old age,
I bring my oil to the car (or truck) dealer and have them put my oil in the
vehicle.

The good news is that both Sam's Club and Costco carry gallons of super duty
diesel engine oil, which is approved for use in your turbo charged Cummins,
Duramax, Mack truck or Caterpillar earth mover.  These oils include Chevron
Delo 400, and Shell Rotella.  Another excellent conventional oil is
Valvoline All Fleet Plus. In the case of my Volkswagon Jetta TDI  diesel
which calls for a 5 W 40 diesel rated lubricant, I use Shell Rotella T
synthetic oil.  Wal-Mart used to carry it but recently  stopped selling it
in my area, so I had to buy it from a local Shell distributor. Two oil
dealers did not know it existed in 5 W 40 (an unusual viscosity for a
diesel - a reflection on the small tolerances of the engine's  manufacture)
and had to special order it for me.  When I picked it up, I asked the oil
distributor if they sold much synthetic oil.  Their response, "No, too
expensive."  Obviously neither they nor their customers understood that oil
is cheaper than metal.

Beware the long oil change interval.  Even though lubricants have gotten
better, and engines are built to closer tolerances, one should be suspect of
the vehicle manufacturer pushing excessively long oil change intervals. [Oil
doesn't wear out, but additives can be exhausted and contaminants can build
up, thereby reducing the oil's lubricity and protective chemistry (acid
neutralization, etc.). Also,  heat can break down the oil, denigrating its
lubricity.]  Remember, this extended oil change interval is more the work of
the marketing department than the engineering department.  These are the
same folks who recommended the lower tire pressure in the Ford Explorer to
soften the ride - which caused heat build up and subsequent tire failure
which, in turn, contributed to (initiated) the rollover problem.

A major reason for the extended oil change recommendation is to get the
vehicle back to the dealer for routine maintenance, where multiple service
activities can be performed at the same time.

I don't mind the extra trip for the oil change, as it is protecting my
investment and its durability and reliability.

Since I have much more confidence in my oil selection than the dealer's, I
bring my own oil and let them change it, "with customer supplied oil".  I
make an appointment and drive the vehicle to the dealer at the scheduled
time and WAIT for it.  This means that the oil will be changed WHILE THE
ENGINE IS HOT - essential for draining all (as much as possible) the
suspended contaminants out of the engine (the main reason for changing the
oil in the first place).

I think you have to balance the oil change interval and associated cost with
the savings in future repairs and higher longevity (and lower long term
cost) of the vehicle.

In the case of my two previously owned Mercedes diesels (purchased new)
they had their oil changed every 2000 to 2500 miles with "earthmover grade"
diesel engine oil.  I out 400,000 to 500,000 miles on them, before I sold
them and never had a significant or expensive engine repair. Yes, oil is
cheaper than metal.

All told, in most cases a diesel will cost less to run and maintain per
mile.  The real question is will the mileage be sufficient to make up for
the additional up front purchase price.  Obviously there are some
intangibles which could factor in to the decision, such as pulling ability
(torque) and miles per tank of fuel.  Diesel fuel is readily available in
most parts of the country - if there are trucks, there are diesel fueling
stations.

I know I may have beaten this subject to death, and I don't mean to insult
anyone's intelligence. However, I think many of the items I have brought up
are not as well known as they should be.

I sincerely hope that the above information is useful to some of you.

Sincerely,

Oliver Filippi