Pressure-sensing glowplugs from Beru and a joint venture between Siemens/VDO and Federal-Mogul are said to be on their way to the diesel market by "mid-2007". That's good timing, considering the new Tier 2 Bin 5 emission standards slated to go into effect for passenger-car diesel in the U.S. next year.
Exhaust gas oxygen sensors, as used in gasoline engines, don't work on diesel due to the fact that they typically run lean (meaning not much information can be extracted from measuring residual oxygen), and of course particulates and other diesel byproducts do the delicate sensors no favors. Indirect combustion-quality sensing methods, such as exhaust gas temperature, can be difficult to correlate to actual combustion performance. (More after the jump...)
The direct sensing of cylinder pressure, however, is the Holy Grail for engine-management engineers. Being able to measure the pressure vs. crank angle during the combustion process will theoretically allow for maximizing mean cylinder pressure (that's good for power) while minimizing peak cylinder pressure (that's good for durability and NOx emissions). It will also help to equalize cylinder-to-cylinder performance, which has traditionally been a problem for diesels.
Now, if only we could get a similar system for spark-ignition engines. That's probably not far behind, and I'm thinking that this technology will prove to be very useful in HCCI engines as well.