High Speed - Low Speed Small Disp Diesels

For a good education on the subject, check out David Pascoe's site:

http://www.yachtsurvey.com/GasDiesel.htm

Here are a few of his thoughts -

"The Problem With Light Weight Engines There is a direct relationship between service life and the weight of engine blocks and cylinder heads. The heavier, or thicker the castings, the longer they will last. That's one of the reasons why older engines just seem to go and go, while we often refer to the engines of recent years as "throwaways."

The problem with light castings is a problem of both strength and heat distribution. Diesel engines, with their 350-550 lb. internal cylinder compression, develop tremendous heat within the cylinders and heads. When castings heat up they expand, and when casting thickness are unequal, this can lead to cracking. It follows then, that the thinner the casting, the weaker it is, and therefore more prone to heat distortion and cracking.

This has been one of the major problems of trying to adapt light weight automotive engines to marine use. Because the loads are much greater, more heat is generated, and therefore more distortion of the castings occur. And when distortion occurs, the close tolerances of the moving internal parts such as crankshaft, bearings and journals, rods, pistons and cylinder walls goes out of whack. The end result is an early demise of the engine. Therefore the move to adapt high speed, light weight small truck engines to marine use results in an engine with a decidedly shorter service life. One of the most common problems that we see with light engines is the frequent cracking of cylinder heads, which is the first place that designers seek to reduce weight."

 

Tran, the difference is essentialy written in the reciprocating engine power equation. It essentially states that the power output of an engine is given by:
Power = K x Pm x D x RPM
where:
- K is a constant depending on engine geometry (comprising efficiencies, multipliers, conversion factors and other things you don't need to consider for this analysis)
- Pm is a mean effective pressure acting on pistons
- D is the displacement of the engine
- RPM is rpm.

Now, the Yanmar in your example has a displacement of 4.2 litres, Perkins has 4.4 litres. That's pretty similar, so you can reduce the equation above and say that, in order to compare those two engines, the power output is proportional to the product of essentialy two numbers, Pm and RPM:
Power = K * Pm * RPM

Since the RPM of the Perkins is just 74% of Yanmar's RPM (2800/3800=0.74), if Pm was equal between two engines you would expect Perkins's power to be 74% of Yanmar's which means 233 HP. Since it is not the case, it means that Perkins' design has a much lower mean effective pressure.

How did Yanmar obtain so much higher mean effective pressure? By doing two things:
1) shorter piston stroke (Yanmar: 100 mm, Perkins: 127 mm)
2) use of turbocharger.

So what is the mean effective pressure? It is a fictitious constant pressure, produced by fuel combustion, which pushes at the piston during the stroke phase of the engine cycle. The actual pressure acting on the piston is highly variable during the cycle, so we adopt this mean pressure to simplify the math.
The point is this:
- a higher Pm means more mechanical stress on the engine structure (that's intuitive, think of thin plastic bottles for soda water vs. thick, heavy equipement needed for scuba diving)
- a higher RPM means more mechanical stress on the engine structure (because of inertial forces).

Since modern fast diesels adopt both design features, they are much more mechanically stressed (or loaded) than the traditional, slow revving and naturaly aspirated engines.
And that's essentialy the physical reason why that old Buckau Wolff engine (post #7 from Apex1) has had (and probably will yet have) a long and happy life, while a modern small fast diesel's lifetime is comparatively short.

At the end, it really all reduces to Fast Fred's concise remark - the difference lies in longevity.

 

thank you very much - I even understood it.

 

"thank you very much - I even understood it."

Aint this board group GREAT!

FF