Algorithm development and analysis for advanced engine technologies including piezoelectric fuel injection and variable valve actuation

Bradley W Pietrzak, Purdue University

Abstract

As vehicle emissions standards and fuel economy constraints become increasingly strict, the automotive industry must employ the use of advanced engine technologies to overcome these challenges. Fuel injection rate shaping and cylinder deactivation (CDA) are two such technologies, and both of them require the design and implementation of algorithms using various hardware and software tools. Fuel injection rate shaping is one path towards cleaner and more efficient diesel engines. Piezoelectrically-actuated fuel injectors are well-suited for rate shaping operation, but are difficult to control. Control-related challenges arise primarily due to the lack of measurements available in a fuel injection system on-engine, the inherent complexity in the dynamics of a piezoelectric fuel injector, and variability from injector-to-injector and over the life of a given injector. Although these challenges are significant, model-based fuel flow rate estimation and control is of the utmost importance due to the fact that the brake torque in a diesel engine is primarily influenced by the total amount of injected fuel per engine cycle. This thesis studies the effect of injector model parameter uncertainties on the model-based estimate of the injector's output fuel flow rate. Specifically, the relationship between the injector's needle seat area and needle lift is investigated. While off-engine experiments can be conducted to determine this parameter, this study presents an on-engine parameter estimation strategy that can accommodate for some of the aforementioned injector variability. In the presence of an initial parameter error of 25\%, the parameter estimator improved the model-based prediction of total injected fuel by approximately 10\% in Matlab simulations. CDA is another technology that enables improved fuel economy and reduced tailpipe emissions in diesel engines. As the name suggests, CDA involves deactivating some combination of an engine's cylinders in order to temporarily reduce the total displacement of the engine. Reduced engine displacement can improve fuel economy and reduce harmful engine emissions (by means of reduced air-to-fuel ratio and reduced pumping work), especially at low engine speed and load conditions. However, there are a few challenges that CDA presents. First, engine lubricating oil can accumulate in deactivated cylinders as time progresses. Second, cylinders may not perform normally immediately upon reactivation (a concept referred to as "first-fire readiness") due to this oil accumulation as well as low in-cylinder temperatures that are the result of a prolonged deactivation. Third, changing the combination of firing cylinders can yield undesirable torsional vibrations during CDA operation. This thesis analyzes the first and second of these issues using in-cylinder pressure measurements to study combustion in cylinders that have been reactivated after prolonged periods of deactivation. Experiments show that as more time is spent in CDA mode, more oil accumulates in deactivated cylinders. This oil accumulation can be as much as 500 mg for cylinders that have been deactivated for 20 minutes. CDA durations of 5 and 10 minutes yield accumulated oil masses of up to 376 mg and 255 mg, respectively, while a CDA duration of 0.5 minutes yields an oil accumulation of less than 1 mg. Since the combustion of this accumulated oil causes abnormally large cumulative heat releases in the engine cycles following the transition from CDA to six cylinder mode, the brake torque does not smoothly transition between these two engine modes. For CDA times of 5, 10, and 20 minutes, these torque fluctuations make such long periods of CDA-only operation unacceptable from a first-fire readiness perspective. Finally, this thesis presents a basic cylinder recharging strategy that can be used in future work to mitigate the effect of oil accumulation and improve first-fire readiness. While improvements in piston ring design can prevent oil accumulation, this cylinder recharging strategy uses software to reactivate all deactivated cylinders for a single engine cycle at regular intervals in an effort to raise in-cylinder pressures enough to prevent oil from seeping into deactivated cylinders. The ability to perform these "recharge events" has been added to the engine test cell used in this study and has been validated experimentally. Although CDA-only operation is unacceptable for periods of time greater than or equal to 5 minutes, CDA operation with regularly-spaced recharge events could enable prolonged CDA operation by mitigating the effects of oil accumulation and first-fire readiness.

Degree

M.S.M.E.

Advisors

Shaver, Purdue University.

Subject Area

Automotive engineering|Mechanical engineering

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