Page 236 - The Tribology Handbook

Pistons B16

GASOLINE ENGINE PISTONS

The key features of a piston for a gasoline engine are shown in Figure 16.4.

Slot Crown 1

Top land

Ring belt Compression
height (H)

Skirt

I
Pin boss
Figure 16.4 Features of a gasoline engine piston

The skirt guides the piston and must be machined slightly out-of-round to compensate for the thermal expansion of the
piston and to ifacilitate hydrodynamic lubrication between the piston and the cylinder bore.
In gasoline pistons in engines of less than 65 bhphitre slots may be used around part of the circumference of the piston to
act as a thermal break between the skirt and the crown, and to introduce more flexibility into the skirt. This allows the
running cieara.nce of the skirt to be minimised so as to improve piston stability.
The Compression height of the piston (H) is governed by the top land height, the ring belt height and the piston pin
diameter. The compression height of gasoline pistons is kept to a minimum in order to reduce reciprocating mass and give
improved powertrain refinement.

Piston weight may be characterised by the apparent
density (K), defined as follows:-

M
K = - g~m-~
03
where M = mass in grams of bare piston (no rings or pin)
D = bore diameter in centimetres.

The relationship between apparent density and compres-
sion height for lightweight designs of gasoline pistons is
shown in Figure 16.5. 0.3 I I I I
0.3 0.4 0.5 0.6 0.7
H
Compression height ratio, -
D
Figure 16.5 The variation of apparent piston density
with compression height for a lightweight gasoline
piston.
B 16.3