I'll take a crack at these...
I'm still slowly reading my way through the paper, but am already struck by a few passages.
Here are a couple...
The compression ratios which permit attaining optimum fuel consumption in small swirl-chamber engines range between 16 and 18.
What do you think about that? Other texts on diesels I have remark that fuel efficiency goes up in NA diesels until the CR is about 26:1, when increasing mechanical stresses overcome further gains. I have to wonder about the effect of turbcharging.
I'd say I've got no reason to doubt the optimal CR's as presented for these motors, but I haven't read other sources that give conflicting information either. Have read some that supports it. I'd be surprised if the optimal CR were above 23:1, because the paper goes into a lot of discussion about "engineering tradeoff" of raising the CR to 23:1 to promote good cold starting and cold idling. I think the optimal CR depends on how much frictional losses increase as the CR increases, which is not the same for every engine. It would be increased on an IDI motor with all the combustion chamber surface area where more heat can be lost.
The answer to this question, and the trade-off being mentioned, is that from a purely theoretical thermodynamic analysis, thermal efficiencies increase logarithmically (i.e. diminishingly) as a function of compression ratio according to roughly
(compression ratio)^(n-1). This "First-Law Analysis" doesn't take heat transfer during compression and expansion into account, but in reality, this cannot be ignored. As compression ratios increase, geometrically the ratio of surface area-to-volume also increases, which therefore increases heat transfer, and reduces efficiency, among other effects.
In an IDI engine, compression ratios are higher than is really needed for optimum thermal efficiency due to cold-starting issues. Jake's paper, and contemporary common knowledge has it that the optimum compression ratio for thermal efficiency is closer to between 16-18:1, and most modern Diesel engines are being designed into this direction. For example, the PD TDI engines have CR between 18-18.5:1, while the state-of-the-art BMW, VAG and M-B Diesels have CRs between 16.5-17.5:1 This range is high enough for good first-law efficiency without excessive heat losses due to the high surface-to-volume ratio.
The 1.5l 50 BHP Diesel and the 1.6l 110 BHP spark ignition engine use the same internal engine components (crankshaft, pistons, conrods and bearings).
So why does it say that diesels are "necessarily" more expensive than their gassers?
Where does it say that? The original 1.5 diesel engine block I think may be from the same casting at the 1.5l gasser, or might even be the same machined part number (not really clear on that), but I think may be different (they talk about reinforcements in certain areas for the diesel?), so I am not sure, would have to compare part numbers. The conrods are I think the same and I wouldn't be surprised if the bearings were as well. It does say that component tolerances are held smaller by selecting the ones most blueprinted for the diesel, also by using multiple head gasket thicknesses. This additional manufacturing complexity seems like it would add to the cost. Other sources of cost difference could be in the fuel injection components (injection pumps, etc) also there is the benefit of no spark system. The pistons are definitely different with a metal reinforcement inside the dieel version that's not in the gas, also a different shape on the surface. The paper goes into great length describing their development. Machining tolerances are higher with the diesel - they even matched pistons to bores according to "honing groups" to achieve the tight piston-wall clearance required in the Diesel. None of this is necessary in manufacturing the gas engine.
The VAG Diesel engines have the same critical feature sizes such as cylinder bore-to-bore spacing, main-bearing diameters and widths, and con-rod big-end dimensions and wrist pin diameters to their gasser breathren, but this does not imply that they're the same part. The first 1.5L Diesels and early 1.6Ds did indeed have SOME identical components like cranks, con-rods and bearings to their gasser breathren, but as the power ratings increased with turbocharging and what-not, components were beefed-up as required.
The early VAG Diesels were not completely troublefree, but they had nowhere the bad reputation as the GM 5.7Ds. This is because VAG recognized that they were already starting out with a stout basis of an engine design. From its earliest days, many VW cranks and con-rods were forgings rather than the cheaper and inferior castings that GM used and still use to this day... VW's excellent modern Diesels -- which still very much have the same roots and resemblances to the `70s' ancestors -- give testament of the basic engine design, and that's why VW has largely resisted any major changes to a winning formula.
As to the cost deltas between Diesels and gassers in general, they do exist. I can list just a few components that Diesels have that gasser's of the same class and power ratings generally don't:
- Turbocharger, intercooler and associated plumbing
- Injection pump and injectors (highly precise components compared to gasser components)
- Under-piston oil jet coolers
- Forged cranks (some VWs, notably the contemporary 2.slow gassers, have gone to cheaper cast cranks :roll: )
- Cooled EGR
- Etc.
Also keep in mind that the price premium you see on the sticker of a car NEVER reflects an automaker's true cost. For example, that $2k price premium you see listed for a Diesel option in a new car truely only costs a fraction of that in terms of the component price differences to an automaker, but they (the OEMs) also have to amortize development expenses and qualifying/certifying powertrains for the markets in which they are to be sold, plus of course the obligatory profit margin.