GB2254372A

GB2254372A - Spark ignition engine piston crown formation

Info

Publication number: GB2254372A
Application number: GB9107235A
Authority: GB
Inventors: Thomas Tsoi-Hei Ma
Original assignee: Ford Motor Co
Current assignee: Ford Motor Co
Priority date: 1991-04-06
Filing date: 1991-04-06
Publication date: 1992-10-07
Also published as: GB9107235D0

Abstract

The circumference of the crown of the piston 10 is provided with projecting heat exchange fins 16 to increase the heat transfer area between the end gases and the piston, the spaces 14 between the fins 16 being sufficiently wide to enable the combustion flame to propagate between the fins without being quenched. The spaces between the fins 16 provide at least 3% of the minimum combustion chamber volume. The roots and tips of the fins are rounded and the fin height is between 0.5 and 3.5 times the fin spacing. <IMAGE>

Description

Title DETONATION SUPPRESSION Field of the invention The present invention relates to the suppression of detonation, which is also termed knocking, in a spark ignited internal combustion engine.

Background of the invention It is believed that engine knocking is caused by spontaneous ignition of the some of the gases in the combustion chamber, ahead of the arrival of the flame front originating at the spark plug. The mechanism is believed to be that the part of the original mixture which burns last by virtue of being furthest away from the spark plug is compressed and heated by the advance towards it of the flame front and can reach a sufficiently high temperature and pressure in the available time to exceed the point at which detonation occurs. This results in uncontrolled burning at the same instant of all the remaining charge in the combustion chamber, which causes a sudden very high pressure rise which is injurious to the engine and is accompanied by a characteristic pinking noise.

Description of Prior Art It is known to detect pinking, which usually occurs at low speed and high load, by a knock sensor which responds to the characteristic vibrations. The spark timing is then retarded to avoid knocking in subsequent cycles. If the compression ratio is adjustable, a reduction in compression ratio will also successfully overcome the danger of knocking. These steps are however remedies for ensuring the engine does not exceed its safe knock limit and they do not address the problem of how the tendency to knock can be reduced in the first place.

A search has revealed the existence of certain prior art references which consider the detonation process in the combustion chamber and attempt to alleviate it. For example, US 2,662,517 and US 2,662,514 and EP 0 157 853 all attribute detonation to an acoustic effect and attempt to modify acoustic propagation in the combustion chamber with the aim of attenuating the effect of the detonation. To this end, these reference described acoustic horns and resonance chambers in the piston which interfere with the sound propagation. These reference are however based on an incorrect understanding of the detonation process and attempt to suppress a symptom of detonation instead of preventing detonation in the first place.

Obiect of the invention The present invention therefore seeks to provide an engine in which the tendency to knock is reduced by designing the combustion chamber to ensure that the end gases do not reach the physical conditions at which detonation occurs.

Summary of the invention According to the present invention, there is provided a spark ignited internal combustion engine in which the circumference of the crown of the piston, at least in its regions furthest from the spark plug, is provided with projecting heat exchange fins to increase the heat transfer area between the end gases and the piston, the separation between the fins being sufficiently wide to enable the combustion flame to propagate between the fins without being quenched.

US 2,662,517 mentioned above has helical fins in the piston top land which are intended to act as an exponential acoustic horn. The gap between these fins tapers to a point and will eventually quench the combustion flame before all the end gases are consumed. In this patent, the risk of detonation is unintentionally reduced because the end gases are cooled by the increased piston surface area, but a significant amount of end gas will remain unburnt at the end of combustion, tending to increase the hydrocarbon emissions of the engine.

The provision of helical grooves surrounding the top piston land is not preferred because it interferes with the heat flow pattern in the piston crown. It is advantageous instead to provide fins projecting upwards from the rim of the top surface of the piston.

It is also preferred that the fins be arranged on radii centred on the spark plug in order to minimise disturbance to flame propagation.

Typically, the dimensions of the fins may be such that they define between them a volume of 3% or more of the volume of the combustion chamber when the piston is at top dead centre.

The fins tend in practice to run slightly hotter that the remainder of the piston crown. While the increased fin temperature reduces the temperature differential between the end gases and the piston, the effect of increased heat transfer area significantly outweighs the importance of this increase in temperature and the net result is to cool the end gas more effectively than a conventional piston design. This cooling furthermore only affects the end gases, which account for less than 10% of the total charge, and does not therefore affect the overall efficiency of the engine.

To avoid excessive hot spots and to improve heat flow between the fins and the remainder of the piston, it is preferred that the roots and the tips of the fins be rounded and that sharp corners be avoided.

In determining the dimensions of the fins, both the height of the fins and their separation will affect engine performance.

If the fins are too high, that is to say much taller than the gap between them, then they will themselves run too hot and ultimately present a risk of pre-ignition. On the other hand, if the fins are shallow, then the increase in heat transfer area will be small and the fins will have little or no effect. In the preferred range, the height of the fins is between 0.5 and 3.5 as great as their separation.

Brief description of the drawings The invention will now be described further, by way of example, with reference to the accompanying drawings, in which: Figure 1 is a plan view of a piston crown of an engine of a first embodiment of the invention, Figure 2 is a section through the piston of Figure 1, Figure 3 is a plan view, similar to that of Figure 1, showing the piston of a second embodiment of the invention, and Figure 4 is a section through the piston of Figure 3.

Description of the preferred embodiments Figures 1 and 2 show a piston 10 the crown 12 of which is slightly convex upwards. Around the periphery, the crown has recesses 14 which form fins 16. The provision of these recesses 14 increases the surface area of the piston in the region of the end gases and tends to cool the end gases, that is the gases furthest away from the spark plug during combustion, to prevent them from igniting spontaneously before the arrival of the flame front. The embodiment of Figures 3 and 4 differ from that of Figures 1 and 2 in that the crown 12' of the piston 10' has a bowl and is concave upwards. The recesses 14'in this case are formed in the rim surrounding the bowl and define fins 16'.

In both embodiment, the spaces between the fins, that is the volume of the recesses must be substantial and must amount to at least 3% of the minimum combustion chamber volume.

In operation, the end gases will tend to collect in the recesses 14, 14' and because of the increased surface area, the cooling of the end gases will be improved without having to cool the entire piston and cylinder. The risk of detonation or auto-ignition will in this way be reduced.

However, the fins will themselves tend to run hotter than the piston surface. The heat flow out of the fins is only through conduction down the roots and if the fins are too tall then they will run hot and increase the risk of preignition. A compromise is therefore required between fin area and fin temperature which can be determined empirically for each engine. Typically the height of the fins should be more than half of their separation to provide a useful increase in surface area and should be less than three and a half times the separation to avoid excessive fin temperature.

To assist in cooling the fins and reduce the risk of flame quenching it is preferred that the tips and roots of the fins be rounded and that sharp edges and corners be avoided.

As the flame propagates radially along the piston after ignition, it is important to ensure that the flame is not quenched by the fins and prevented from entering the recesses as this will cause incomplete combustion. This is avoided by ensuring that the dimension of the recesses exceed the quench distance (approximate 1.5 mm) of the flame.

The radial disposition of the fins is to minimise the disturbance to the heat flow in the piston and thereby maximise the heat flow away from the fins.

Claims

1. A spark ignited internal combustion engine in which the circumference of the crown of the piston, at least in its regions furthest from the spark plug, is provided with projecting heat exchange fins to increase the heat transfer area between the end gases and the piston, the separation between the fins being sufficiently wide to enable the combustion flame to propagate between the fins without being quenched.

2. An engine as claimed in claim 1, wherein the fins project upwards from the rim of the top surface of the piston.

3. An engine as claimed in claim 1 or 2, wherein the fins are arranged on radii centred on the spark plug in order to minimise disturbance to flame propagation.

4. An engine as claimed in any preceding claim, wherein the dimensions of the fins are such that they define between them a volume of 3% or more of the volume of the combustion chamber when the piston is at top dead centre.

5. An engine as claimed in any preceding claim, wherein the roots and the tips of the fins are rounded, sharp edges and sharp corners being avoided.

6. An engine as claimed in any preceding claim wherein the height of the fins is between 0.5 and 3.5 as great as the-separation between the fins.

7. An engine constructed, arranged and adapted to operate substantially as herein described with reference to and as illustrated in the accompanying drawings.