Page 197 - 4637

operating at low rpm and low bhp these engines are among the
most economical engines today. The only engine comparable from
the standpoint of economy is the spark plug engine fueled with low
priced natural gas. And this type fuel is not always available, espe-
cially in wildcat drilling.
The second type, because of its low unit weight and low
weight/bhp is well suited to rigs working in shallow fields where
moves are frequent. Their operational cost/bhp day is not so good
as heavier engines but they have increased flexibility and
maneuverability.
Diesel Lubrication. Lubrication poses a more difficult
problem than lubrication of spark plug jobs. The compression ratio
being higher, blow by is harder to prevent. Using a heavy fuel
there is greater formation of carbon and dilution of crankcase
because of fuel oil leaking past the piston rings. Frequently diesel
fuel is found to contain sulfur. This creates sulfuric acid in the
crankcase unless the crankcase temperature is kept high.
Diesel lube oil today is additive or detergent oil. It is usually
manufactured with three levels of detergency. For use with a good
fuel on normal loads oil with 5% additives might be satisfactory.
Oil containing 10% additives is generally considered better. If bad
fuel is used in conjunction with heavy loading, an additive content
of 20% may be advisable. If there is a question of doubt, get a
good lubrication engineer's advice.
As can be seen from the above, oil changes must be quite
frequent. Some operators change oil as often as every three days.
Few go longer than a week between changes.
Dual Fuel Carburetion. This type of fuel control allows a
diesel engine to operate on 100% diesel fuel or part diesel oil and
part natural gas. Theoretically a mixture containing 10% diesel
fuel oil will give combustion within the cylinder, that is, will
produce the diesel cycle. Actually a diesel explosion may be
produced with as low as 5% diesel oil content.

196