The following is a reply I posted in a thread at TDIClub entitled, "
DI vs. IDI, interesting article posted on the GTD forum." I thought it would elicit good debate here.
http://vwdiesel.net/phpBB/viewtopic.php?t=2329
Basically, to summarize...
An IDI with a VNT and an intercooler actually can be more efficient and powerful than a TDI.
It's just that heat loss problem, and it looks like they're working on fixing that, too.
Apparently the IDIs produce less NOx, too.
Hmm...
Small correction: an IDI can potentially match the performance of the TDI on a displacement basis. But it will always incur a fuel consumption penalty because the inherent nature of the design has greater heat transfer losses and throttling losses though the transfer port between the swirl- and main combustion chambers.
BUT! Going back to performance, the trend in Diesel engine design over the past few years has been to reduce compression ratios for two reasons: the main one has been that increased power ratings of the engines necessitate reducing the CR to maintain peak cylinder pressures to levels that can be withstood by the engine structure. Secondly is that compression ratios have been higher than is actually optimal for best efficiency, in order to offset the need for good cold-starting and low-load, low-speed operation characteristics.
DI can run just fine with CRs down to as low as 16:1. Toyota's 2.2L 180HP D4-D engine has a CR of only 15:8:1. Large truck, locomotive, and marine engines can tolerate even lower compression ratios and have still greater thermal efficiencies than small Diesel engines with higher compression ratios.
On the other hand, IDI, because of it's heat losses, cannot run well at compression ratios as low as DI. There can be some mitigating design solutions, like the use of ceramics and low-heat-rejection, insulating materials around the combustion chamber. Notwithstanding, however, IDI will still need substantially higher CRs, which means for a given peak cylinder pressure (PCP) limit, which is dictated by the design of the engine structure, you will get to a point where the IDI Diesel will hit a wall. Again, this is not because of limitations of the combustion process, but by thermal and gas pressure stress considerations. As far as PCP goes, the current state-of-the-art for automotive engines stands at around 180 - 200 bar. A moderately tuned TDI on the stock compression ratio and slightly increased boost and developing 300 lb.ft. of torque would already achieve these PCP levels. Therefore, in the interest of durability that is expected in an OEM engine, manufacturers would not further develop IDI technology unless there are breakthroughs in PCP or the ability to run lower CRs with everyday performance, driveability and efficiency that customers would expect.
Of course, that's not going to stop the guys like those at vwdiesel.net from pushing the outputs of the IDI engine ever higher. But the state-of-the-art of DI engine development is not sitting still, either. No one 10 years ago would have imagined that a TDI engine you can buy today would have almost DOUBLE the HP (170 vs. 90) from the factory with only a 0.1L increase in displacement, and in modified form be an eyeshot from making 280 HP/400 lb.ft. An IDI would be very hard pressed to reliably match this. 400 lb.ft. with a 22.5:1 compression ratio, and the boost pressure to support the fuelling at this output level, however you slice is, results in tremendous gas pressure forces and PCPs, and would cause some serious pretzel bending of rods among other badness.
Topic: IDI engine, there IS a future... (Read 83395 times)
