The Hindu Business Line : Blending diesel benefits into petrol performance

UNDERSTANDABLY, diesel (compression ignition, or CI) engines have gained significant and well-deserved attention for their superior and efficient performance at all speeds and loads, and their potential for reduced environmental impact.

The biggest challenge will be the Euro 5 looming large over the next decade, requiring drastic reduction of carbon dioxide and other emissions. This is possible to achieve, although the control measures are expensive. According to a European survey, the percentage of diesels in the market will continue to grow.

CI engines have a bright future. Petrol (spark ignition, or SI) engines, on the other hand, have shown more potential and room for development by cross-fertilisation of certain dominant characteristics of diesels at competitive cost. Many unexplored areas of improvement can be exploited by the use of high-tech electronic control. The Table, showing the comparative characteristics of petrol and diesel engines, reveals the crucial causes for the weakness of a SI engine.

The major deficiency of the SI engine, restricting its ability to equal its diesel counterpart in fuel efficiency, is its inability to take advantage of the high compression ratio on account of knock limitation. Recall that the trend is reversed in the CI engine, where the higher the compression ratio, the higher is the efficiency! Hence, the designer fixes an optimum compression ratio, consistent with the knock-limited power, at the load and speed over which the vehicle is likely to be driven.

The second crucial deficiency is the method of load control by the use of a throttle controlling the quantity of the mixture but not the quality or the air-fuel ratio as in the case of the diesel. The advantage of leaning the fuel in the mixture, which leads to greater fuel efficiency, is denied to the gasoline engine.

Fluid friction losses encountered by restrictions in breathing, reduce the volumetric efficiency and hence the output. Moreover, the exhaust temperature of the engine is uniformly high and does not decrease at part loads as the quality of the mixture, and consequently the temperature, is constant at all loads.

Consequently introducing the features of high compression ratio and load/speed control of the diesel engine would be beneficial to the gasoline engine, which would then have the advantage of throttle-less control. This is the origin of the stratified charge or lean-burn concept introduced in the modern gasoline engines by using heterogeneous regions of rich mixtures around the spark plug and progressively leaning the mixture in regions remote from the sparkplug in response to load and speed changes.

Smart engine concept

These inherent deficiencies of petrol engines are being corrected using high-tech electronics control. "Smart engines" are being designed with in-built "intelligent (i)" electronic functions.

This permits variable-valve timing and lift electronic control (i-VTEC) and variable cylinder management (VCM), with facilities to deactivate selected cylinders. Thus the engine will be able to give power on demand, fuel consumption being consistent with the speed and load on the vehicle.

When selected cylinders are deactivated, the intake and exhaust valves are shut off while cutting the fuel simultaneously. While the cylinders idle, VCM controls the ignition timing. The drive-by-wire (DBW) system adjusts the torque converter system, reulting in the levelling-up of the torque surges resulting from deactivation of cylinders. Multi-Displacement System (MDS), is the most significant new technology.

In other innovations, a continuously variable compression ratio (VCR) facility is provided such that the operation is automatically adjusted to the "Highest Useful Compression Ratio" at all the loads and speeds encountered, as vehicles cannot be driven at fixed loads and speed. Contrast this with fixed compression ratio engines, where the CR is only optimised to the highest load and speed conditions. This big handicap for an automobile petrol engine can now be overcome by the use of continuously variable compression ratio (VCR) technology.

FEV Engine Technology, Inc., is working on this advanced technology and hopes it can get close to the fuel efficiency of a diesel engine by this technology development. This concept is based on the eccentric crankshaft bearing. The rotation of the eccentric by a 36 or 42 volt electric motor, in either direction, permits the centre line of the crankshaft with the piston assembly to move up or down relative to the cylinder head — thus altering the compression ratio.

This is achieved by electronic control, based on feedback from appropriate sensors in the cylinder, which detects the highest useful compression ratio consistent with the continually varying load and speed dictated by the driving cycle. CNG vehicles, which are derived from the diesel engines powering off-highway and other heavy vehicles, can profit by incorporating several of these features.

The emergence of several successful new engine designs is illustrative of the tendency to produce high performance and low emission hybrid engines. Gasoline injection (multi-point MPI or single point SPI) is one step towards convergence of the two technologies since you are lending to a carburetted engine the injection benefits natural to diesel technology, although the injection is limited to manifold below the throttle body (SPI) or in the ports leading to the cylinder (MPI).

The next stage of evolution is the orbital engine concept, in which the injection of gasoline at appropriate timing takes place by compressed air into the cylinder of a ported two-stroke engine (like the good old air-injection diesel of Dr Rudolf Diesel's time, excepting that injection occurs here after the exhaust-ports close following the bottom dead-centre.)

This is a perfect example of fusion of the good points of a high-torque two-stroke spark-ignited petrol engine getting rid of the much-blamed camshaft and valves. Of course, there must be a separate provision for supplying compressed air for injection during running and starting. The camless valve train is now the new approach as electronics dominates the automotive industry.

There is no necessity for compressed air in the direct injection stratified charge (DISC) or gasoline direct injection engines (GDI). The injection is by separate timed solenoid actuators from a pressure chamber into the cylinder with a broad range for injection timing, starting from the beginning of the intake stroke (unlike the timing of the diesel engine just before the compression stroke) to match the performance and low emission requirements. But the sparkplug is the ignition source, not the heat of high compression ratio.

There are reports of piston damage by overheating and other problems created by vaporisation of volatile components of the gasoline resulting in phase separation during injection. These are the problems that innovative designers face at present. Now Siemens VDO automotive has developed injection by piezo-electric actuators for diesel engines and this is being considered for gasoline direct injection to mitigate some of these problems.

Another refinement in the attempt to match the diesel engine's performance is to use variable valve timing with electronic control (VTEC) using double overhead camshafts actuating four valves per cylinder and exclusively two cams for each valve for low- and high-speed ranges. This shift of cams is achieved by electronic control on the hydraulic shifting system.

Both gasoline and the diesel engines have their individual advantages. A good blend of the diesel engine features of fuel economy and performance in all driving conditions, with the cost-effectiveness and drivability with power and torque of the gasoline engine would be a welcome fusion of both technologies without sacrificing the clean emission standards.

Such an approach is the focus of increasing research efforts. According to the former chief of Volkswagen, this fusion of technologies is called the Combined Combustion System (CCS). With the advent of hybrid electric vehicles, there is need for a light-weight vehicle, and the Wankel rotary engine, which is receiving renewed attention, is considered the best option.

(The author is a former Professor of Mechanical Engineering, Indian Institute of Technology, Madras. Feedback may be sent to: bsmurthy@saeindia.org)

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