Reducing Pumping Loss

Reductions in flow pressure loss can be achieved by reducing the pressure drop that occurs in the flow of air (air fuel mixture) into the cylinder, and the combusted mixture through the exhaust system. The largest part of pumping loss during normal driving results from throttling, however, and strategies to reduce throttling loss have included variable valve timing, “lean-bum” systems, and “variable displacement” systems that shut off some engine cylinders at low load.

Intake manifold design

There are various strategies to reduce the pressure losses associated with the intake system and exhaust system. Efficiency can be improved by making the intake air flow path as free as possible of flow restrictions through the air filters, intake manifolds, and valve ports.45 Intake and exhaust manifolds can be designed to exploit resonance effects associated with pressure waves similar to those in organ pipes. By properly tuning the manifolds, high pressure waves can be generated at the intake valve as it is about to close, which increases intake pressure, and at the exhaust valve as it is about to open, which purges exhaust gases from the cylinder. Formerly, “tuned” intake and exhaust manifolds could help performance only in certain narrow rpm ranges. Recently, the introduction of new designs, including variable resonance systems (where the intake tube lengths and resonance volumes are changed at different rpm by opening and closing switching valves) have allowed smooth and high torque to be realized across virtually the entire engine speed range. Manufacturers expect variable intake systems to be in widespread use over the next 10 years.

Multiple valves

Another method to increase efficiency is by increasing valve area, especially by increasing the number of valves. A four-valve system that increases flow area by 25 to 30 percent over twovalve layouts has gained broad acceptance. The valves can be arranged around the cylinder bore and the spark plug placed in the center of the bore to improve combustion. While the peak efficiency or brake-specific fuel consumption (bsfc) of a four-valve engine may not be significantly different from a two-valve engine, there is a broader range of operating conditions where low bsfc values are realized. Analysis of additional valve layout designs suggests that five valve designs (three intake, two exhaust) can provide an additional 20 percent increase in flow area, at the expense of increased valvetrain complexity. Current expectations are that most engines will be of the four-valve types by 2005.

Under most normal driving conditions, throttling loss is the single largest contributor to engine efficiency losses. In SI engines, the air is throttled ahead of the intake manifold by means of a butterfly valve that is connected to the accelerator pedal. The vehicle’s driver demands a power level by depressing or releasing the accelerator pedal, which, in turn, opens or closes the butterfly valve. The presence of the butterfly valve in the intake air stream creates a vacuum in the intake manifold at part throttle conditions, and the intake stroke draws in air at reduced pressure, which results in pumping losses. These losses are proportional to the intake vacuum, and disappear at wide open throttle.

Lean-burn

Lean-bum is one method to reduce pumping loss. Instead of throttling the air, engine power can be reduced by reducing the fuel flow so that the air-fuel ratio increases, or becomes leaner. (In this context, the diesel engine is a lean-bum engine). Most SI engines, however, do not run well at air: fuel ratios leaner than 18:1, as the combustion quality deteriorates under lean conditions. Manufacturers provided data on engines constructed to create high swirl and turbulence when the intake air and fuel are injected into the cylinder that can run well at air: fuel ratios up to 22:1. Lean-bum engines actually run at high air-fuel ratios only at light loads; they run at stoichiometric or rich air: fuel ratios at high loads to maximize power. The excess air combustion at light loads has the added advantage of having a favourable effect on the polytropic coefficient, n, in the efficiency equation. Modem lean burn engines commercialized recently in Japan do not completely eliminate throttling loss, but the reduction is sufficient to improve vehicle fuel economy by 8 to 10 percent. A disadvantage of lean-bum engines, however, is that they cannot use conventional three-way catalysts to reduce emissions of nitrogen oxides (NOX , and the in cylinder NOx emission control from running lean is sometimes insufficient to meet stringent NOx emissions standards. There are developments in “lean NOX catalysts,” however, that could allow lean-bum engines to meet the most stringent NOX standards proposed in the future, which will be discussed later.

Variable valve timing

Variable valve timing (VVT) is another method to reduce pumping loss. Instead of using the butterfly valve to throttle the intake air, the intake valves can be closed early, reducing the time (and volume) of air intake. The system has some problems at very light load (the short duration of the intake valve opening leads to weaker in-cylinder gas motion and reduced combustion stability). Moreover, at high rpm, some throttling losses occur at the valve itself.47 Hence, throttling losses can be decreased by 80 percent at light load, low rpm conditions, but by only 40 to 50 percent at high rpm, even with fully VVT.

Aside from improved fuel economy, VVT also increases power output over the entire range of engine rpm. Fully variable valve timing can result in engine output levels of up to 100 brake horsepower (BHP)/liter at high rpm without the decline in low-speed torque that is characteristic of four-valve engines with fixed valve timing. In comparison to an engine with fixed valve timing that offers equal performance, fuel efficiency improvements of 7 to 10 percent are possible. The principal drawback has historically been the lack of a durable and low cost mechanism to implement valve timing changes. Honda has commercialized a two stage system in its four valve/cylinder engines where, depending on engine speed and load, one of two valve timing and lift schedules are realized for the intake valves. (This type of engine burn to achieve remarkable efficiency in a small car.)

Another version of VVT also shuts off individual cylinders by example, an eight-cylinder engine can operate at light load as has been combined with deactivating the valves. a four-cylinder engine lean For (by deactivating the valves for four of the cylinders) and as a six-cylinder engine at moderate load. Such systems have also been tried on four-cylinder engines in Japan with as many as two cylinders deactivated at light load. At idle, such systems have shown a 40 to 45 percent decrease in fuel consumption, while composite fuel economy has improved by 16 percent on the Japanese 10-15 mode test since both pumping and fictional losses are reduced by cylinder deactivation. 50 Earlier systems had problems associated with noise, vibration, and emissions that resulted in reduced acceptance in the market place, but more recent systems introduced in Japan have solved most of the problems. OTA had the opportunity to drive Mitsubishi’s MIVEC V-6 which features VVT and cylinder shutoff, and noise and vibration effects on this vehicle from cylinder shutoff were barely noticeable.

Total effect

All of the aforementioned technologies can reduce pumping loss, increase volumetric efficiency, increase specific output, and reduce fuel consumption at part load, but the benefits are not additive. Most manufacturers provided estimates of benefits for several combinations; for example, a recent paper by engineers from Porsche forecast a 13 percent reduction in fuel consumption with no loss in performance for a system featuring variable valve timing and lift, variable resonance intake, and cylinder cut off (from a baseline vehicle featuring a four-valve engine with a two-stage resonance intake and cam phase adjustment) . This estimate is more optimistic than what many manufacturers believed to be possible.