Part Load Pumping Losses in an SI Engine

This article is the first in a series of articles on the benefits of throttle-less operation in a spark ignited (SI) engine. Throttle-less operation centres on using a variable valve lift system rather than a throttle to limit the air flow into the combustion chamber. This page looks at some of the basics associated with part load operation in an SI engine. In particular, it describes how conventional throttling affects part load fuel economy by increasing pumping losses.
In automotive applications, spark ignition (SI) engines operate over a wide range of different speed and load conditions. For the majority of the time, they operate below their rated output whilst cruising or idling, and are only required to produce their maximum output intermittently during acceleration or climbing an incline.
Operation below the full rated load at a given engine speed is termed part load operation, and requires the engine output to be restricted in order to maintain a given engine speed.

Part Load Pumping Losses

Part load operation of SI engines is conventionally achieved by the use of a throttle to restrict the airflow into the engine, hence allowing the quantity of fuel that is injected to be reduced, whilst maintaining a constant air-fuel ratio (AFR).
The ratio of the trapped air mass to the maximum mass of air at its intake density that could be contained in the cylinder is termed the volumetric efficiency. When operating under full load conditions, the volumetric efficiency of an IC engine should be as high as possible so that the mass of air-fuel mixture, and hence the power output, is maximised.
Engines are therefore designed to minimise the restriction of air flowing into the engine, so that the air can be drawn into the cylinder as close as possible to atmospheric pressure.
When operating at part load the throttle restricts the airflow into the engine, reducing the volumetric efficiency, and as a result the air pressure in the intake manifold falls significantly below atmospheric pressure. In order to draw air from the manifold into the cylinder, the piston is required to do work against the manifold depression and this is termed pumping work (Strictly speaking, the work done by the piston is a result of the pressure differential between that of the manifold and the crankcase).

Air Standard Cycles

The air standard Otto cycle is illustrated in figure 1 (see top right), whilst figure 2 (see right) illustrates a more typical PV diagram for an SI engine operating at part load.
It can be seen from the comparison of the two diagrams that the ideal cycle has no pumping loop (grey shading). This indicates that the gas exchanges from the intake manifold into the cylinder, and from the cylinder into the exhaust manifold after combustion, should ideally occur without any associated losses. In practice, this can never be realised, and work is always expended drawing air into the cylinder and expelling exhaust gas from the cylinder.
When operating at full load, the intake manifold will be slightly below atmospheric pressure, and the exhaust manifold will be slightly above atmospheric pressure. As the engine power requirement is reduced, the throttle is required to reduce manifold pressure further below atmospheric pressure, hence increasing the size of the pumping loop as illustrated in figure 2.
The work output of an IC engine is indicated by the difference in area contained within the power loop and the pumping loop. As engine output is reduced therefore, the area of the pumping loop becomes closer to that of the power loop and hence the majority of the positive work produced by the engine is being utilised to overcome pumping and frictional losses.

P-V Diagrams Ideal vs Throttled

This situation causes throttled SI engines to exhibit very poor efficiency under part load conditions compared to their efficiency under full load operation.
The part load pumping losses partially account for the poor economy of SI engines when compared to diesel engines, which do not require throttling to operate at part load.
If the area of the pumping loop can be reduced for a given engine output, less work will be expended in the gas exchange process and a corresponding reduction can be made in the area of the power loop. The reduction in the size of the power loop indicates a reduced fuel requirement and hence improved efficiency will result.

How We Can Improve The Situation?

This concludes our introduction to part load operation. In part two we will describe how we can use a variable valve lift and duration system to change our part load operating strategy and therefore improve fuel economy.
Part 2 – Intake closing strategy