Reducing Pumping LossReductions 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 two-
valve 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.^46 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
(^45) The shaping of valve ports to increase swirl in the combustion chamber can lead to reduced volumetric efficiecy, leading to a tradeoff between
combustion and volumetric efficiency. 46
K. Aoi et al., “Optimization of Multi-Valve Engine Design: The Benefit of Five-Valve Technology," SAE paper 860032, 1986.