18.6.
Lean-burn Engine Control
To reduce CO emissions efficiently use of more fuel-efficient engines, such as lean-burn engines, which potentially offer greater fuel efficiency, may gain importance. Operating an engine with lean-of-stoichiometric mixtures (A/F ratios of 16:1 – 25:1) requires burning of fuel in an excess of oxygen and this can provide fuel consumption improvements of at least 10%, in addition to substantial reductions in CO and NOx emissions. Since a lean-burn engine operates close to the limits of combustion, its efficient functioning demands good mixture preparation, a very high energy spark to ignite the weak mixture and very good monitoring of combustion quality and mixture strength through a closed-loop control system. Many of these concepts are incorporated into recent Japanese designed engines, namely Honda’s VTEC-E and Toyota’s Carina-E. These engines can typically run at A/F ratios of 22:1, meet European and US emission regulations and offer fuel consumption improvements of up to 25% under cruising conditions.
The most important component of their control systems is a type of EGO sensor (called a universal exhaust gas oxygen sensor or UEGO sensor), which delivers a variable output current in proportion to exhaust oxygen content.
18.6.1.
Operation of the UEGO Sensor
The UEGO sensor is an advancement of the conventional heated zirconia EGO sensor. In addition to detecting the stoichiometrric point this sensor can also measure a wide range of A/F ratios from very rich (10:1) to very lean (35:1).
One type of sensor, shown in Fig. 18.19, contains two oxygen transfer cells such as an oxygen pumping cell (Ip cell) and an oxygen detecting cell (Vs cell). A small constant current, fed to the V& cell moves a tiny amount of oxygen to the right so that the O2 cavity is filled with oxygen. This oxygen acts as a ‘reference gas’ for the sensor.
Fig. 18.19. Construction and output characteristics of the universal exhaust gas oxygen sensor.
As exhaust gas enters the detecting cavity, a voltage is developed across the Vs cell, depending upon the oxygen concentration in the exhaust gas. Theip cell then controls the oxygen concentration in the detecting cavity by pumping oxygen to, or from, the atmosphere so that the Vs voltage is maintained at a steady value of 0.45 V. The pumping current, Ip is, therefore, a measure of the air-fuel ratio of the exhaust gas.
18.6.2.
Combustion Monitoring
In-cylinder sensors provide important data on the timing and quality of the combustion process, which is specifically important when engines are operated with lean mixtures or heavy EGR. Presently available techniques used to evaluate these data are direct pressure measurement with a pressure sensor (used on some Japanese-market vehicles) and indirect monitoring through spark-plug ionization current. The spark-plug voltage ion method detects the ion density within the combustion chamber by measuring the decay time of voltage after a spark. Using this technique, the limits of leanburn or EGR can easily be detected.
Another much cheaper method is to derive a combustion quality value from piston acceleration. To obtain this the crankshaft speed fluctuations are measured using the crankspeed sensor.
18.6.3.
Toyota Lean-burn Engine
A lean-burn engine control system developed by Toyota (Fig. 18.20) uses in-cylinder sensors to monitor combustion pressure, and a sequential injection system to control fuelling on a cylinder-by-cylinder basis. The A/F ratio for the engine is monitored through a wide-range oxygen sensor installed in the exhaust down-pipe.
Fig. 18.20. Lean-burn engine control system used on a Toyota engine.
A novel feature incorporated is the use of a swirl control valve (SCV) mounted in the intake system. The intake tract for each cylinder is divided into two passages. One passage is smooth, which allows maximum gas flow for good cylinder charging, and the other passage is fitted with a corkscrew-shaped flange, which introduces swirl into the incoming air. The engine ECU can switch air flow in one of the two tracts by actuating the SCV. The engine operates in a lean burn mode when it is running under light to medium load conditions. The ECU directs the SCV to open-up the curved inlet passage to have well mixed incoming vapour due to a high level of turbulence through swirl. During combustion, the ECU monitors the pressure sensor signals for any signs of slow or incomplete burning and then appropriately modifies the A/F ratio or ignition timing on a cylinder-by-cylinder basis. Because of the continuous monitoring of the exhaust gas oxygen concentration, these facilities permit the engine to produce high efficiency at A/F ratios upto 25:1.
When the engine runs under high load conditions, specifically during overtaking or when hill climbing , the ECU switches to stoichiometric operation for maximum power. To achieve this the ECU directs the SCV to open the smooth intake passage and uses the oxygen sensor signal to maintain an A/F ratio of 14.7:1.
