Quote:
Originally Posted by JCL
But other things aren't equal. I agree that a lower speed engine will have potentially less cylinder bore wear, but recall that the gasoline version is now turbocharged and gets its peak torque at a similar rpm to the diesel. My 535i reached peak torque at 1300 rpm, and I would expect the V8 to be somewhat similar.
Now add in the peak cylinder pressures from the diesel compared to the gasoline version. This will impact block fatigue, bearing loads, and thust loads on the piston and cylinder wall.
I know that piston loads increase with the square of engine speed, and only linearly with increased cylinder pressures, but the diesel always has higher cylinder pressures, and the gasoline engine is only occasionally at higher rpm.
All in all, I would call it a wash. There is no logic to the diesel lasting longer, but it shouldn't last any less time, either. And it isn't like the transmission, ancillaries, and electronics are still going to be working as new when the engine finally does give out.
|
One would expect that both the gas and diesel blocks, bearings, etc, would be designed accordingly with those cylinder pressures in mind. But even if the new tubocharged gas engine reaches its torque peak at a similar rpm to the diesel, the fact remains that the siliconized cylinder liners (gas) have historically had lower durability than cast iron (diesel).
Another mark against the gas is the trouble BMW has had with ethanol blended fuel when used in direct injected gas engines (335i). At least in the US, that 10% blend will likely become 25% incrementally in the next 10 to 15 years. A blown HPFP with other collateral damage at higher mileages may send it the junkyard.
The biodiesel OTOH has better fuel properties (cetane, lubricity) than regular D2 so that looks like a plus for future diesel use.
This argument really is academic though in that most BMW owners will not keep their car past 4-5 years, much less 10-15. And those that do will likely have their pocketbook lightened by something else than an engine failure.