Diesel combustion plays out very differently from the majority of spark-ignited engines. The injected fuel must mix with air
in the combustion chamber and absorb enough heat to vaporize and burn. This period is known as ignition delay.
It was going to happen eventually; closed-loop combustion control has finally been introduced to the diesel world. Since
the first electronically controlled diesel engines appeared in the mid-80s, diesels have been required to operate in an open-loop
environment. This was not a problem at the time, because it was relatively easy to meet existing emission control standards
without using feedback from the combustion process. These earlier engines relied first on ECM calibrations and then catalysts
to bring their emissions into compliance.
Diesel cylinder pressure sensing can be accomplished by integrating a pressure sensor and associated electronics into the
glow plug assembly. Combustion pressure for individual cylinders can then be monitored in real-time during the entire combustion
cycle, and this data can be utilized to adjust injection timing, injection quantity and EGR operation.
While it is still possible to build emission-compliant diesels without monitoring what is actually taking place in the combustion
chamber, the costs involved continue to rise. Diesel engines have always cost more than gasoline engines. However, the price
difference tends to increase as emission control regulations become stricter. In order to sell a diesel-powered passenger
car in all 50 states, the vehicle must meet Tier II Bin 5 (also known as T2B5) emission standards. T2B5 is tough on both NOx
and particulate matter (PM) emissions, and has become the "holy grail" for vehicle manufacturers that want to increase their
diesel presence in the automotive marketplace. Heavy reliance on catalysts can get the job done, but this approach is expensive
as the price of precious metals such as platinum and rhodium has risen significantly in the past decade.
Another major cost challenge in building today's clean diesels is the production tolerances of emission-related components.
All modern light-duty diesels now utilize high-pressure common rail injection systems, and these require extremely tight tolerances
in order for the system to perform properly. Tighter tolerances increase production costs, making diesels more expensive and
thus more difficult to sell to the cash-crunched consumer. The diesel's superior fuel economy means little when both the price
of fuel and the cost of the vehicle are higher than that of its gasoline counterpart.
The world's first production vehicle using cylinder pressure sensing is the 2009 Volkswagen Jetta TDI diesel. The 2009 Jetta
uses Beru/Sensata PSG technology to meet Tier II Bin 5 emission standards and has been certified for sale in all 50 states.
Closed-loop combustion control can be used to decrease diesel engine production costs and improve drivability and emissions
performance. "Closed-loop" means that information is gathered about what is taking place during the combustion event, and
this data is then used to modify engine management system operation. In gasoline engines, this is typically accomplished using
oxygen sensors that are located in the exhaust stream. In the case of diesel engines, however, cylinder pressure data can be viewed to gain an accurate view of the combustion process. This information can then be used to modify engine
control module (ECM) outputs such as injection timing, injection quantity and EGR system operation, as well as enhancing diagnostic
functions such as the misfire monitor.
Tony Martin is an associate professor of Automotive Technology at the University of Alaska Southeast in Juneau, Alaska. He holds Canadian Interprovincial status as a Heavy-Duty Equipment Mechanic. He also holds 18 ASE certifications, including CMAT, CMTT, L1 and L2.
Articles by Tony Martin
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