CA2178012A1 - Piston - Google Patents

Abstract

A piston for use in an internal combustion engine.
The piston has a combustion chamber in its upper surface.
The combustion chamber is generally cylindrical with radiused upper edges and lower edges, where the walls of the chamber meet the base of the chamber. A combination of a cylinder head having a flat surface to define a top for a combustion chamber and a piston having a combustion chamber in its upper surface is also described. There is a clearance between the piston top and the cylinder head at top dead center that defines a squish band. The clearance of the cylinder head is 1.75% of the piston diameter. The squish band width is about 16.7% of the piston diameter.

Description

PISTON

This invention relates to a piston.

There has been great concern in recent years to produce internal combustion engines that have low emissions. Striking success has been achieved but the number of vehicles in use worldwide is constantly increasing. As a result, the atmospheric pollution produced by internal combustion engines remains at about the same level as it was some years ago.

Liquified petroleum gases (LPG) are considered of major interest in reducing atmospheric pollution. LPG is seen as the preferred alternative fuel to gasoline. In general LPG comprises propane or butane. Liquified natural gas has also been used as a fuel.

LPG has a great advantage of low cost and low emissions. In an engine having no facility to treat exhaust gas LPG has substantially lower emission levels than a gasoline engine.

The market for LPG systems for heavy duty vehicles is small compared to that for light duty vehicles. LPG
is quite widely used in taxis and city buses. Private passenger cars have also been converted but heavy duty vehicles have not used LPG as a fuel as much.

There are a variety of systems for using LPG for combustion engines. The so called first generation systems use a carburetor with a shut-off valve, an evaporator/pressure regulator and a mixer. The kit was typically added as a retro-fit and was installed in dual fuel vehicles able to run on gasoline and LPG. The electro magnetic shut-off switched between LPG operation and gasoline operation, depending on engine requirements.

' 2178012 The second generation of LPG carburation systems came into effect because of increasingly stringent emission requirements. In general the second generation is an improvement over the first generation by the application of electronic control devices. The metering of the gas flow is controlled electronically in the second generation systems.

Subsequent developments comprise the use of fuel injection, which is markedly more efficient than carburation and more susceptible to electronic control.
The vast majority of new vehicles have engine management systems that control fuel supply, ignition and exhaust treatment. These highly sophisticated systems, developed principally for gasoline engines, have actually made it more difficult for LPG conversion. In particular, combustion can occur in the inlet manifold using LPG, causing damage to the injection system and the inlet air system. To avoid this disadvantage several LPG equipment manufacturers have used multipoint LPG injection systems.
These systems are microprocessor controlled, self learning and do not require manual adjustment. They may be considered the third generation of LPG equipment.

As indicated LPG technology has found limited use in heavy duty vehicles. The use can either be as mixed diesel-LPG or LPG alone. For mixed use a diesel engine is not modified. The diesel combustion principal is maintained and a second fuel system is added that handles the LPG. The amount of diesel fuel used is reduced and the original performance level maintained by adding LPG
to the intake air. Ignition is initiated by the diesel fuel and continues with the LPG. Although of interest the amount of diesel fuel replaced by LPG in this way is limited. For extended use, for example in a city bus, the average replacement is about 25~.

A second approach is to convert the diesel engine to an Otto engine. This means that diesel use is no longer possible and only LPG is used as fuel. The complete diesel fuel system, the fuel pump and fuel lines and injectors are removed. The cylinder head is adapted so that spark plugs can be fitted and the shape of the piston is also changed because of the lowered compression ratio. An LPG carburation or, more usually nowadays, an LPG injection system is fitted. Adjustments are also made to the speed governor.

As a result of this progress LPG vehicles with third generation LPG equipment have at least as low emission level as modern gasoline engines. That standard is extremely high compared with, say, 20 years ago but it is felt that there is room for improvement and, for example, in California increasingly stringent standards are applied.

It is believed that a particular advantage of LPG is at cold start, particularly at lower ambient temperatures. Gasoline evaporates relatively poorly at low ambient temperatures and gasoline engines must be choked or enriched for a warming up period. The use of fuel injection on these engines has reduced the time for which choking is required but an enriched supply of gasoline must still be made if the vehicle is to be driven under cold starting conditions. This leads to high engine emissions of carbon monoxide and hydrocarbons. The typical catalytic converters used on modern vehicles cannot handle these emissions because of the lack of air that is used to catalytically oxidize the emissions.

LPG does not have a problem with evaporation. LPG
is, of course, used under pressure and its release into a manifold causes immediate evaporation of the liquid. No enrichment of the air/fuel ratio is necessary.

With a view to improving and extending the use of LPG on commercial vehicles the applicants have modified a diesel engine to enable it to run on propane. The engine was converted to spark ignition as described above. A
throttle and a new fuel system were added to supply the propane fuel to the engine. The fuel is injected in liquid form into the intake airstream between the turbocharger, common on diesel engines, and the throttle.
Liquid propane is used to avoid phase change problems at low temperatures. Furthermore the properties of liquid propane change less with temperature, at least in the range in which the engines operate, than those of gaseous propane. This makes engine control simpler.

It is a particularly important feature that the engine is operated on a lean air/fuel mixture as this minimizes exhaust emissions and reduces heat rejection problems normally found with conversions of diesel engines to spark ignition. The running of an engine on a lean mixture is, of course, well known and modern engine management systems, controlling a fuel injection system, are a great deal more efficient in this regard than even the best carburettors. Good control and mixing of the fuel and air are required. Control is important due to sensitivity of exhaust emissions to air fuel ratio that the lean mixtures use. Good mixing is needed to ensure that the correct mixtures are delivered to each cylinder.
Furthermore the combustion chamber of the engine must be correctly designed to burn the mixtures efficiently.

The present invention is particularly concerned with the design of the combustion chamber in a diesel engine converted to spark ignition and designed for propane 217801,2 fuel. The invention is concerned to improve gas flow and thus good mixing of the air and fuel.

Accordingly the present invention, in a first aspect, provides a piston for use in an internal combustion engine having a combustion chamber in its upper surface.

The combustion chamber is generally cylindrical with chamfered or radiused upper edges and radiused lower, internal edges.

The invention also provides a combination of a cylinder head having a flat surface to define a top for a combustion chamber and a piston having the combustion chamber in its upper surface.

There is a small clearance between the head and the top of the piston known as the squish band. In the present invention the squish band width is about 16.7~ of the cylinder bore diameter. The clearance of the cylinder head is 1.75~ of the bore diameter.

The squish band is important. It generates turbulence in the combustion chamber as the piston approaches top dead center. The turban has improved the engine's ability to burn lean mixtures completely, thus reducing emissions.

The invention is illustrated in the single drawing, which is a partial section through a piston 10 according to the present invention.

Figure 1 shows a piston 10 having a conventional opening 12 to receive a wrist pin. The piston 10 has a combustion chamber 14 formed in its upper face 16.

~ 217801,~

The upper edges of the chamber 14 are radiused or chamfered at 18. The lower corners 20, where edges 22 of the chamber meet base 24 of the chamber, are also radiused. Typically the upper edges 18 have a radius of about 4.7mm and the corners 20 have a radius of about 12.7mm.

Typically the chamber 14 is dimensioned to achieve a compression ratio of about 9.2 to 1. The combustion chamber 14 is generally machined into the top of the piston 10. The remaining top surface 16 of the piston has a small clearance to the underside of a cylinder head 26 called the squish band.

The height 'h' of the squish band is about 1.75~ of bore (or piston) diameter 'd'. The width 'w' of the squish band is about 16.7~ of the diameter 'd'.

The depth of the chamber 14 and the width of the squish band are chosen to provide turbulence at the chosen compression ratio. All other things being equal, a combustion chamber with high turbulence will operate more consistently at leaner air fuel ratios. Thus localized build-up of one component is avoided.

Although the foregoing invention has been described in some detail by way of illustration and example for purposes of clarity of understanding, it will be readily apparent to those of ordinary skill in the art in light of the teachings of this invention that certain changes and modifications may be made thereto without departing from the spirit or scope of the appended claims.

Claims (5)

1. A piston for use in an internal combustion engine and having a combustion chamber in its upper surface.

2. A piston as claimed in claim 1 in which the combustion chamber is generally cylindrical with radiused upper edges and lower edges, where the walls of the chamber meet the base of the chamber.

3. In combination a cylinder head having a flat surface to define a top for a combustion chamber and a piston having a combustion chamber in its upper surface, a clearance between the piston top and the cylinder head to define a squish band.

4. A combination as claimed in claim 3 in which the squish band width is about 16.7% of the piston diameter.

5. A combination as claimed in claim 3 in which the clearance is about 1.75% of the piston diameter.