My engineering experience is not in diesel engine design. I would also like to see an efficiency comparison between the various direct and indirect injection designs. Here is a excerpt from a reference written by Chevron which describes the difference between direct and indirect diesel engines. (I think John Miller III originally posted a link to the original paper)
The two fuel injection processes used in diesel engines, direct-injection (DI)
and indirect-injection (IDI), are illustrated in Figures 6-2 and 6-3. In a DI
engine, fuel is injected directly into the cylinder above the piston. In an IDI
engine, fuel is injected into a small prechamber connected to the cylinder via
a narrow passage that enters the prechamber tangentially. During the compression
process, air is forced through this passage, generating a vigorous swirling
motion in the prechamber. Then fuel is injected into the prechamber and ignition
occurs there. The combination of rapidly swirling air in the prechamber
and the jet-like expansion of combustion gases from the prechamber into the
cylinder enhances the mixing and combustion of the fuel and air.
The more rapid mixing of fuel and air achieved in IDI engines comes at a price,
however. The high velocity flow of air through the narrow passage connecting
the main cylinder to the prechamber, as well as the vigorous swirling motion
in the prechamber itself, causes the air to lose significantly more heat during
compression than it does in a DI engine. Coupled with a pressure drop from
the main chamber to the prechamber, this results in an air temperature in the
prechamber after compression that is lower than that in a similar DI engine.
Since rapid fuel autoignition requires a certain air temperature, an IDI engine
needs a higher compression ratio to achieve the desired air temperature in
the prechamber. IDI engines operate at compression ratios of about 20:1 to
24:1; while DI engines operate at ratios of about 15:1 to 18:1. The heat losses
that necessitate these higher compression ratios have another, more important
effect: they decrease the efficiency of the engine. IDI engines typically
achieve fuel efficiencies that are 10% to 20% lower, on a relative basis, than
comparable DI engines.
Even with the higher compression ratios, IDI engines may still be hard to start.
Most IDI engines use glow plugs to heat the air in the prechamber in order to
make starting easier. Glow plugs, which are small resistive heaters, are usually
powered for only the first few minutes of engine operation.
With the negative attributes of harder starting and lower efficiency, one may
wonder why IDI diesel engines are used at all. The answer is engine speed. As
an engine gets smaller, generally it must operate at higher speeds to generate
the desired power. As engine speed increases, there is less time per engine
cycle to inject, vaporize, mix, and combust the fuel. As a result, the higher
mixing rates afforded by IDI designs become necessary to achieve good
combustion at higher engine speeds. IDI diesels most commonly are used in
smaller automotive and light duty truck applications.
As to the question of a clutch on a hydrostatic, I can’t quite agree with your assessment. My understanding of a hydrostatic transmission is that there is a variable displacement pump piped to a fixed displacement motor, which in turn drives the output shaft of the transmission. If the hydrostatic controls are in neutral, then there is no oil being pumped, and thus no (or very little) additional load on the starter. My suspicion is that the foot clutch on the Kubota is a hold-over from their gear drive units to allow a disconnect for the PTO. The Deere 4000 series (except for the 4100) use a separate hand activated clutch for PTO engagement, which is entirely independent from the propulsion system. A much cleaner design in my opinion. Feel free to disagree. /w3tcompact/icons/smile.gif
Computers don't make mistakes.... What they do, they do on purpose.
