Pistons for internal combustion engines

A piston (10) for a diesel engine is formed with a combustion bowl (21) which has a lesser volume until the cylinder pressure reaches that at which fuel ignition takes place. The volume of the combustion bowl is then increased to reduce the maximum pressure in the cylinder. The volume is again decreased during the expansion stroke. This has a number of beneficial effects on the engine including reducing the stress on the engine parts and allowing increased level of pressure-charging.

BACKGROUND TO THE INVENTION 
1. Field of the Invention 
The invention relates to pistons for internal combustion engines and more 
particularly to pistons for diesel engines. 
2. Review of the Prior Art 
Since the fuel in a diesel engine is ignited by the temperature of air 
compressed in the cylinder prior to injection of the fuel, it is 
necessary, if combustion is to take place, to compress the air by a 
predetermined amount to ensure that the required temperature for fuel 
ignition is reached. The fuel is, however, injected before a piston in a 
cylinder of the engine reaches top dead centre and so the pressure in the 
cylinder continues to rise after the fuel has been injected. 
In view of the high compression ratios used in diesel engines, the maximum 
cylinder pressure, reached, at or shortly after top dead centre, can be 
substantial. This peak pressure imposes loads on the piston which can 
damage bearings and reduce the effectiveness of lubrication. In addition, 
it can cause shock waves to pass through the engine block which can in 
turn cause cavitation in water cooling systems which leads to erosion of 
metal from the water side of the cylinders. The rate at which the pressure 
rises also causes fatigue and cracking in the piston and reduces the life 
of the gudgeon pin bosses. Further, the high rate of pressure rise is an 
important factor in the noise emission spectrum of diesel engines. The 
maximum pressure also determines the amount by which the air can be 
pressurised before entry into the cylinder and affects adversely the 
equipment for injecting the fuel. In addition, in certain cases, it makes 
the use of a heater necessary on starting the engine. 
SUMMARY OF THE INVENTION 
According to the invention there is provided a piston for an internal 
combustion engine and comprising a crown formed at least partially by a 
member movable relatively to the remainder of the piston, the member 
moving in each compression stroke, when the pressure in the associated 
cylinder reaches a predetermined level, from a first position to a second 
position in which the combustion chamber volume is increased, the member 
maintaining said second position until a predetermined pressure is reached 
on each expansion stroke, when the member returns to said first position.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Referring first to FIG. 1, the first form of piston comprises a piston body 
10 cast from aluminum or aluminum alloy and formed with a skirt 11 and 
piston ring grooves 12. The centre of the body is provided with a 
cylindrical bore 13 terminating at its lower end in an annular radially 
extending surface 14 provided with a central hole and an annular step 15. 
The surface 14 is covered with a steel plate 16. A crown 17, of an 
aluminum or ferrous alloy, is fixed to the body portion 10 by bolts 18 and 
includes an outer surface 19 contiguous with the skirt 11 and formed, 
optionally, with a further piston ring groove 12. The crown 17 is 
generally annular and is provided with a radially inner annular surface 20 
which forms an entrance to a combustion bowl 21. An annular bore 22 leads 
from the radially inner edge of the surface 20. The diameter of the bore 
22 is less than the diameter of the bore 13 in the piston body so that a 
step 23 is formed between the two parts. 
A generally cylindrical piston-like member 24 is carried by the piston body 
10 and is formed with an upper surface 25 which forms the central portion 
of the combustion chamber 21. The member 24 has a cylindrical outer 
surface 26 and is a sliding fit in the crown bore 22. The surface 26 
terminates in an annular rabbet 27 which, in the position of the member 24 
shown in FIG. 1, is in engagement with the step 23 between the crown 17 
and the piston body 10 to prevent axial movement of the member 24 in an 
upward direction. The cylindrical surface 26 is provided with grooves 28 
which receive sealing rings (not shown) for forming a seal between the 
member 24 and the crown 17. 
The member 24 also includes a lower portion 29 of generally cylindrical 
configuration which is received within the bore 13 in the piston body 10 
and which has a diameter substantially less than the diameter of that 
bore. Within the annular gap between the lower portion 29 and the bore 13 
are arranged grouped pairs of plate springs 30, the uppermost bearing 
against the member 24 and the lowermost bearing against the steel plate 
16. The arrangement is such that the plate springs 30 are held under 
partial compression or preload when the piston is in the position shown in 
FIG. 1. 
The piston is assembled by inserting the plate 16 into the piston body 10 
and then placing the plate springs 30 on the plate 16. The member 24 is 
then inserted with the cylindrical part 29 extending down through the 
washers into abutment with the plate springs 30. The crown 17 is then 
placed in position so that the step 23 presses down on the rabbet 27 of 
the member 24 thus partially compressing the plate springs 30. The bolts 
18 are then inserted to fix the crown 17 to the piston body and hold the 
plate springs 30 under the partial compression. 
In use, the piston is inserted into a cylinder of a diesel engine which may 
be a two-stroke or four-stroke diesel engine and may be either naturally 
aspirated or preferably pressure-charged. As is well known, a diesel 
engine works on the principle of compressing a charge of air to a 
temperature at which diesel fuel will ignite and then, when the requisite 
compression has been reached, injecting the fuel to produce an expansion 
and exhaust stroke followed by recompression and combustion. The fuel is 
injected before top dead centre. In order to achieve the necessary air 
temperature to initiate combustion, the compression ratio of the engine 
must be much higher than that in an Otto cycle engine; for example, 
between 12 and 18:1 with direct injection engines. 
After the fuel has been injected, and combustion commenced, the pressure 
increases to reach a maximum pressure just after top dead centre, before 
declining in the expansion stroke. This is shown schematically by the 
broken line 31 in FIG. 2 for a conventional piston not including the 
member 24. The high maximum pressure has a number of adverse results 
amongst which are the high stressing of bearings and engine parts, the 
breaking down of lubrication films, the production of shock waves passing 
through the engine block which in turn can cause cavitation in a water 
cooling system leading to erosion of the metal of the engine, and the 
inability of the engine to withstand high boost for a long periods. 
Further, this maximum pressure has an adverse effect on the fuel injection 
equipment, may cause difficulties in starting, and in the attainment of 
acceptable noise emission and vibration spectrums. 
The piston described above with reference to FIG. 1 operates in the 
following way. As the piston commences the compression stroke to compress 
the charge of air, the member 24 remains in the position shown in FIG. 1 
so that the combustion bowl has a lesser volume. The piston is thus acting 
in the same way as a conventional piston to compress the air to the 
temperature necessary to cause combustion of the fuel. Fuel injection then 
takes place and the plate springs 30 are so preloaded, that, up to the 
pressure reached at fuel injection, no movement of the member 24 has 
occurred. At this point, however, the cylinder pressure acting on the 
combustion bowl surface 25 of the member 24 is sufficient to compress the 
plate springs 30 to cause the member 24 to slide in the bore 22 in the 
crown 17 to a second position (not shown) in which the combustion bowl 21 
has a much greater volume. The lower end of the member 24 bears aginst the 
portion of the plate 16 radially inwardly of the step 14. The plate 16 
thus prevents the plate springs 30 and the member 24 wearing away the 
piston body. 
The result of this is that the minimum volume of the combustion chamber in 
the cylinder is increased and the maximum pressure reduced. Once the 
piston has passed top dead centre and maximum pressure, the cylinder 
pressure reduces until the limiting pressure is once again reached. The 
member 24 then moves back to the first position shown in FIG. 1 in which 
the rabbet 27 abuts against the step 23 and the combustion bowl 21 has its 
lesser volume. 
This is shown schematically in FIG. 2 in the continuous line 32. It will be 
seen that the curves 31, 32 follow one another until the fuel injection 
point is reached. The cylinder pressure is then reduced in comparison with 
the conventional piston until a balancing pressure is reached once again 
when the two curves virtually regain coincidence. 
The piston of FIG. 1 has the following advantages: 
1. The peak cylinder pressure is reduced together with the rate of pressure 
rise following injection. This leads to increased bearing life through 
reduced loading while assisting in the maintenance of a satisfactory oil 
film at the crank pin bearing and gudgeon pin bearing surfaces. 
2. The cyclic torque characteristics of the engine are improved and some of 
the energy absorbed around top dead centre is subsequently yielded up on 
the expansion stroke. Thus the specific output of the engine will remain 
equal to or slightly better than that of an engine not using the piston 
described above with FIG. 1, for a given level of fuel input. Thus the 
need for reduction in specific output to produce smoother and quieter 
combustion is obviated and there is less shock vibration in the engine 
structure. 
3. The intensity of shock waves is reduced, leading to a decreased tendency 
to cavitation erosion in water cooled cylinder liners. 
4. Because of the increased volume of the combustion bowl 21, the surface 
25 should have a longer life with less combustion bowl edge erosion. In 
addition, gudgeon pin life should be increased by the reduced pressure. 
5. The engine will stand higher levels of pressure-charging for longer 
periods, providing cooling facilities are adequate. 
6. Such a piston may simplify fuel injection equipment and the cost of such 
equipment and, in some cases, may allow the elimination of use of a heater 
on starting. 
7. There may be an ability of the engine to accept differing fuels without 
redesign. 
8. The compression ratio is unaffected at starting and so there will be no 
adverse effect on the starting characteristics of the engine. 
The space within which the springs are located may be supplied with oil, 
both to cool the springs and to damp the movement of the member 24. 
It will be appeciated that the piston need not be provided with a 
combustion bowl 21. The piston could have a conventional flat crown with 
the member 24 opening up a recess when the predetermined pressure is 
reached in closing the recess when the cylinder pressure drops below the 
predetermined pressure. It will also be appreciated that the piston need 
not use plate springs 30, any suitable spring means such as coil springs 
may be used. 
Two embodiments using coil springs are shown in FIGS. 3 and 4. In FIG. 3, 
the piston is constructed generally as the piston described above with 
reference to FIG. 1 and parts common to FIG. 1 and FIG. 3 will be given 
the same reference numerals and will not be described in detail. In the 
piston of FIG. 3, the member 24 is provided on its under surface with a 
central annular recess 32 and the steel plate 16 is formed with a central 
upward boss 33. Two co-axial coil springs 34, 35 are arranged between the 
member 24 and the steel plate 16. The inner coil spring 34 engages the 
recess 32 and the boss 33 and the outer coil spring 35 engages outer 
portions of the member 24 and the plate 16. The coil springs 34, 35 have 
opposite hands. 
The piston of FIG. 3 operates in the same way as the piston of FIG. 1 and 
has the same benefits. The provision of two coil springs 33, 34 allows an 
increased force to be applied to the member 24 as compared with the FIG. 1 
piston. This may be desirable in certain diesel engines where particularly 
high pressures are generated. 
Referring next to FIG. 4, the second form of piston comprises a crown 40 
and a body 41 both formed from aluminum or an aluminum alloy. The crown 40 
is generally annular and is provided with a ring band 42 including three 
piston ring grooves 43. The centre of the crown 40 is provided with an 
annular surface 44 which forms an entrance to a combustion bowl 45. An 
annular bore 46 leads downwardly from the inner edge of the surface 44. 
The body 41 comprises a generally frusto-conical central portion 49 
connected by bolts 50 to the crown 40. The upper end of the body is formed 
with spaced projections 47 which are drawn against the crown 40 by the 
bolts 50 and which are angularly spaced to form slots between them. 
Aligned gudgeon pin bores 51 are provided and the lower end of the central 
portion 49 is connected to an annular skirt 52 having an upper surface 
lying in a plane normal to the piston axis. A shaped connecting rod 53 has 
a gudgeon pin bore 54 aligned with the gudgeon pin bore 54 in the body 41 
and connected thereto by a pin 60. 
A generally cylindrical piston-like member 55 is a sliding fit in the crown 
bore 46 and is formed with an upper surface 56 which forms the central 
portion of the combustion bowl 45. The member 55 has downwardly and 
outwardly extending spider arms 57 which pass through the slots between 
the projections 47 on the body 41 and carry at their ends an annulus 62 
which engages beneath a lower surface 58 of the crown 40. Thus the member 
55 is free for sliding movement within the crown bore 56 relatively to the 
crown 40 and the body 41. 
A partially compressed coil spring 59 is arranged between the annulus 62 
and a washer 61 provided on the skirt 52. This urges the member 55 into 
the position shown in FIG. 4. 
The piston described above with reference to FIG. 4 operates in the same 
way as the pistons described above with reference to FIGS. 1 to 3 and has 
the same benefits. In comparison with the pistons of FIGS. 1 to 3, the 
piston of FIG. 4 is of light-weight and the increased length of the spring 
59 allows it to bear against the member 55 with increased force. 
The combustion bowl, where provided, can have any required shape. 
Although the pistons of FIGS. 1 to 4 have been described in relation to a 
diesel engine, it will be appreciated that they may be used in an Otto 
cycle engine or any other type of engine.