GB2447445A

GB2447445A - Modifying a diesel engine with a pre-combustion chamber to operate on natural gas

Info

Publication number: GB2447445A
Application number: GB0704677A
Authority: GB
Inventors: Romano Artioli
Original assignee: TAXICRYO Ltd
Current assignee: TAXICRYO Ltd
Priority date: 2007-03-10
Filing date: 2007-03-10
Publication date: 2008-09-17
Also published as: GB0704677D0

Abstract

A method of modifying a diesel engine to operate on natural gas includes the steps of replacing the diesel injector of the engine with a natural gas injector (45, figure 6) and mounting a spark plug 30 within an existing diesel pre-combustion chamber 25 in the cylinder head 1 of the engine, the spark plug 30 igniting the natural gas fuel within a specific combustion chamber in the engine. The spark plug 30 may be mounted laterally with respect to a cylinder; alternatively it may be mounted such that the angle of the spark plug 30 to the vertical cylinder axis is 25-30{, and preferably 27{. The vertical plane angle of the spark plug 30 to the longitudinal axis of the engine may be 65-75{ and preferably 70{. The natural gas injector (45, figure 6) may be mounted close to the intake valve 15 of the cylinder. A sensor may be mounted in the exhaust manifold to monitor the quantity of gas to be provided to the combustion chamber. The pistons of the engines may be replaced with pistons 65 provided with an eccentric well 75 on the upper surface, and the spark plug 30 may be aligned with this well 75.

Description

IMPROVEMENTS IN OR RELATING TO INTERNAL COMBUSTION

ENGINES

This invention relates to improvements in or relating to internal combustion engines and more particularly to a method of modifying a diesel powered internal combustion engine into a natural gas powered engine and to a diesel powered engine modified to run on natural gas.

Diesel powered engines are the most efficient prime movers commonly available today. Many vehicles, particularly heavy duty vehicles and heavy use vehicles are powered by diesel engines as they exhibit improved power, performance, fuel economy and durability over engines within a similar size range.

Diesel engines offer the possibility of combining very high thermal efficiencies with very low emissions, and their good fuel efficiency results in low carbon dioxide emissions. However, the main problem areas for diesel engines are emissions of nitrogen oxides (NOx) and particulates and these two pollutants are traded against each other in many aspects of engine design.

Very high temperatures in the combustion chamber help to reduce the emission of soot but produce higher levels of nitric oxide (NO).

Lowering the peak temperatures in the combustion chamber reduces the amount of NO produced but increases the likelihood of soot formation. The NO produced rapidly oxidises to NO2 (collectively called NOx). NOx combines with hydrocarbons or volatile organic compounds in the presence of sunlight to form low level ozone.

High up in the stratosphere, ozone shields the earth from harmful ultraviolet rays. At ground level however, ozone formed when nitrogen oxides and hydrocarbon emissions combine in the presence of sunlight irritates the respiratory system causing coughing, choking and reduced lung capacity. Urban ozone pollution has been linked with increased hospital emissions for respiratory problems such as asthma and even at low levels, ozone presents hazards for healthy adults.

Environmental pollution problems are only expected to increase over time as vehicle population and vehicle miles travelled increase. Diesel emissions may contribute to some degree to the development of cancer, cardiovascular and respiratory health problems, pollution of air, water and soil, reduction in visibility and global warming.

Concerns over the pollution problems have lead to increasingly stricter emissions regulations particularly in heavily populated areas such as city centres. Where instituted, control programs have been effective in reducing diesel emissions. Fuel changes such as reduced sulphur and aromatics content have resulted in immediate improvements across the entire on-road diesel fleet.

Technology advances such as engine modifications to introduce exhaust gas recirculation or catalytic aftertreatment can take longer to have an effect but are important in reducing the overall effect of diesel pollution.

The stricter emissions regulations and rising fuel costs are forcing operators to seek cleaner and more efficient alternatives.

Natural gas as a vehicle fuel has been seen as one solution to this problem. Natural gas is readily available in most parts of the world and due to local sources is significantly cheaper than diesel in many parts of the world. With the increased availability of inexpensive natural gas, many fleet owners and other operators are now seeking to modify the diesel engines of their vehicles to burn natural gas.

There are several additional advantages to the use of natural gas in automobiles. Methane is the simplest hydrocarbon, hence it is much easier to mix uniformly with air than diesel. Therefore, combustion in engines that are driven by natural gas is more complete, leading to lower carbon-monoxide and hydrocarbon emissions.

In a diesel engine, the fuel ignites spontaneously shortly after injection begins. During this delay, the fuel is vaporising and mixing with the air in the combustion chamber. Combustion causes a rapid heat release and a rapid rise of combustion chamber pressure. The rapid pressure rise is responsible for the diesel knock that is very audible for some diesel engines.

However, methane has a high Octane number (125-130) and therefore possesses good anti-knock characteristics so it can be used with a high compression ratio which results in significantly high thermal efficiency.

The modifications required to transform a diesel engine to a natural gas burning engine have in the past been considered to be a deterrent to fleet owners as the modifications are both time consuming and expensive and obviously the vehicle is off the road whilst the modification is carried out thereby having a further economic knock-on effect to the business of the fleet owner.

It is therefore an object of the present invention to provide a method of modifying a diesel engine to a natural gas burning engine which is simpler and more time efticient than previously known modifications.

According to one aspect of the present invention there is provided a method of modifying a diesel engine to operate on natural gas including the steps of replacing the diesel injector of the engine with a natural gas injector and mounting a spark plug within an existing diesel pre-combustion chamber in the cylinder head of the engine, the spark plug igniting the natural gas fuel within a specific combustion chamber in the engine.

By utilising the existing diesel pre-combustion chamber, the spark plug will be laterally mounted with respect to each cylinder of the engine.

Where the engine has four cylinders, four spark plugs will be mounted, each laterally with respect to the combustion chamber of a cylinder.

Utilizing the original location of the pre-combustion chambers in the cylinder head of the engine allows the spark plugs to mounted in existing apertures in the cylinder head without having to machine additional apertures which would lead to increased down time and increased costs.

Conveniently, the natural gas injector is installed as close as possible to the intake valve of the cylinders.

Preferably the modification method further includes the step of mounting a sensor in the exhaust manifold of the engine to monitor the quality of the gas to be provided to the combustion chamber.

Advantageously, the modification method further includes the step of mounting the spark plugs within the existing diesel pre- combustion chamber such that the angle of the spark plugs to the vertical cylinder axis is about 25 to 300. More preferably this angle is 27 .

Preferably also, the modification method further includes the step of mounting the spark plugs within the existing diesel pre- combustion chamber such that the vertical plane angle of the spark plugs to the longitudinal axis of the engine is about 65 to 75 . More preferably, this angle is 700.

Preferably the modification method further comprises the step of replacing the pistons of the engine with pistons which are provided with an eccentric well on an upper surface thereof.

Advantageously the modification method further comprises the step of aligning the well in the upper surface of the piston with the laterally mounted spark plug.

This maximises the combustion of the natural gas fuel withsn the chamber.

According to a second embodiment of the present invention there is provided an engine modified in accordance with the first aspect of the invention in which each spark plug is mounted within the existing cavity formed as a diesel pre-combustion chamber.

Preferably, the spark plugs are mounted laterally with respect to the combustion chamber of the cylinder of the engine.

Preferably the spark plugs are mounted at an angle to the vertical cylinder axis of about 25 to 30 . More preferably this angle is 27 .

Preferably also, the spark plugs are mounted at an angle to the vertical plane angle of the spark plugs to the longitudinal axis of the engine is about 65 to 750 More preferably, this angle is 70 .

Advantageously the pistons mounted in the cylinders are provided with an eccentric well in an upper surface thereof.

Preferably, the laterally mounted spark plugs are aligned with the eccentric well of a piston within the combustion chamber.

One embodiment of the present invention will now be described with reference to the accompanying drawings in which: Figure 1 is an underside view of the cylinder head of a diesel engine modified in accordance with a first aspect of the present invention; Figure 2 is a part section view on line A of Figure 1 of a spark plug mounting of the modified engine; Figure 3 is a section view of the spark plug mounting of Figure 1 with a spark plug in place; Figure 4 is a side elevation of the intake manifold of the engine modified in connection with one aspect of the invention; Figure 5 is a partial cross section on the line A of Figure 4; Figure 6 is a perspective view of a gas rail and gas injectors which are mounted to the modified intake manifold of the engine; Figure 7 is a schematic view of view of the modified front (crankshaft) pulley of the engine; Figure 8 is a plan view of a piston of the engine modified in accordance with one aspect.of the present invention; Figure 9 is a part cross section on the line A of Figure 8; Figure 10 is a part cross section on the line B of Figure 8; Figure 11 is an underside view of the piston of Figure 8; Figure 12 is a schematic view of an electronic control unit for the modified engine, and Figure 13 is a schematic view of an electronic throttle body which replaces the original mechanical throttle body of the engine.

A method of modifying a diesel powered engine to run on natural gas in accordance with one aspect of the present invention will now be described.

Prior to modification, the engine is a standard 4 stroke diesel engine with air drawn into the piston chamber through the intake valve in the first stroke and then compressed within the chamber in the second stroke. A measured quantity of diesel is then injected into the piston chamber in the third stroke and ignited by the heat of the compressed air within the chamber. Exhaust gasses are expressed from the piston chamber through the exhaust valve in the fourth and final stroke of the engine.

The engine described in the specification is a Nissan 2.7 TD engine which is particularly suitable for powering taxi vehicles and finds widespread use in London in powering black taxi cabs. However, the modifications described can be applied to other diesel powered engines and the invention should not be considered limited only to such an engine as described in the preferred embodiment.

The engine is removed from the vehicle and disassembled. The engine components which will be retained are inspected for wear and damage and reconditioned in accordance with known techniques which will not be further described.

The diesel injection system of the engine comprising the injection pump, injection lines, injectors, glow plugs etc, is disassembled and is not returned to the modified engine.

The cylinder head, injection manifold and front pulley of the engine are modified in accordance with one aspect of the present invention and the modification of each of these components will now be described.

Turning to Figure 1 there is shown a schematic underside view of the cylinder head 1 of a Nissan 2.7 TD four cylinder engine modified in accordance with one aspect of the present invention. The cylinder head includes inlet and outlet ports 5, 10 associated with each piston chamber (not shown) of the engine with valves 15,20 mounted in the ports for reciprocal actuation by the camshaft (not shown).

The existing diesel pre-combustion chamber 25 of the cylinder head is re-machined to house a spark plug 30 for use in igniting the natural gas fuel to be supplied into the piston chambers. The existing steel insert for the diesel pre-combustion chamber is removed and a cast iron pressed cylinder 35 is mounted in its place in the same chamber as shown in Figure 3. A spark plug is mounted into the pressed cylinder for supplying a spark to the air/natural gas mixture to be compressed within the combustion chamber.

As a natural gas engine runs hotter than a diesel fuelled engine, the position of the spark plug in relation to the chamber is critical in maximising the combustion of the air/natural gas mixture within the chamber during the combustion stroke.

As shown in Figure 1, the spark plug 30 is not centrally located in the original diesel pre-combustion chamber and is therefore off-set with respect to the combustion chamber of the cylinder with which it is associated.

Mounting the spark plug in the original diesel pre-combustion chamber rather than drilling a fresh chamber for the plug reduces the overall time taken for the engine modification considerably. This also impacts directly upon the cost of modifying the engine particularly where, such as in the example shown, the engine has four cylinders therefore four new mounting chambers would normally have to be drilled in the cylinder head.

The spark plug vertical plane angle to the engine longitudinal axis in the embodiment shown (represented by the line L in Figure 1) is between 65 to 75 and most preferably 70 . The angle of the spark plug to the vertical cylinder axis represented by a in Figure 1 is between 25 and 30 and most preferably 27 .

Mounting of the spark plug in such an off-set position compensates for the lateral mounting of the spark plug and assists in maximising the combustion of the air/natural gas mixture as will be described further below.

Figure 4 shows the modified intake manifold 40 of the engine which incorporates the natural gas injectors 45 for introducing natural gas into the combustion chambers of the cylinders during the first stroke of the engine. The inlet manifold is adapted to be mounted on the side of the engine block (not shown) with the injectors mounted as close as possible to the combustion chambers.

The natural gas injectors 45 which may be of a known type are mounted on an injector rail 50 as shown in Figure 6 which is in turn mounted on the intake manifold. AS with the injectors, the injector rail may be of a known type.

Figure 7 shows a schematic view of the modified front (crankshaft) pulley 55 of the engine with the addition of a phonic wheel 60 mounted for co-operation with the top dead centre sensor (not shown). The phonic wheel passes a signal which is sent to an electronic control unit of the engine (as will be described further below) representing the angle of displacement of the front pulley thereby enabling the electronic control unit to control the engine timing by opening and closing the natural gas injectors and igniting the spark plugs at the appropriate times.

In addition to modifying various components of the original diesel powered engine as described above, some components need to be replaced with redesigned components and these will be described further below.

Figures 8-11 show the modified pistons 65 which are mounted in the cylinders (not shown) of the engine and together with the cylinders, define the combustion chambers (not shown) of the engine.

The upper surface 70 of the piston is provided with an eccentric well which has a diameter of around 65mm and a depth of around 119mm. The well is provided with a circumferential chamfered lip 80.

The angle of the lip is around 45 to the vertical axis of the well and has a depth of around 2.5mm.

In the preferred embodiment, the centre C of the eccentric well is horizontally offset from the centre of the piston by 8mm and vertically off set from the centre of the piston by 1 mm.

The eccentric well 75 in combination with the cylinder in which the piston 65 reciprocates, defines the combustion chamber for the engine. The eccentric well is positioned to take advantage of the lateral mounting of the spark plugs 30 within the original diesel pre-combustion chamber and thereby has the ability to provide a better compression rate for the natural gas fuel.

The well is positioned on the piston to complement the off-set positioning of the spark plug within the original diesel pre-combustion chamber of the cylinder head. By ensuring that the spark plugs are mounted as close as possible to the combustion chambers, combustion of the natural gas within the combustion chamber is more efficiently achieve and controlled.

The piston 65 is connected to the crankshaft of the engine (not shown) by a connecting rod (not shown). The position of the piston relative to the connecting rod can be adjusted in order to change the operating parameters of the engine.

Electronic control means 85 shown in Figure 12 are mounted to the engine to control the injection of natural gas from the gas rail 50, through the natural gas injectors 45 and into the cylinders for combustion. The control means ensures that the correct amount of natural gas is delivered to each injector at the start of the intake stroke.

The electronic control means controls multi point sequential, phase controlled port injection of the air/natural gas mixture into the combustion chambers of the engine.

An electronic throttle body 90 as shown in Figure 13 replaces the original injection pump control lever of the engine in order to transform the vehicle to a "drive-by wire" system in which the mechanical linkage between the accelerator pedal of the vehicle is replaced with an electronic control to translate the position of the accelerator to a signal passed to the electronic control unit.

The electronic throttle body may be a standard after-market component.

Natural gas components such as a compressed natural gas pressure regulator, filter, gas lines and a natural gas tank and safety components (not shown) are fitted on the vehicle.

The natural gas tank may be mounted within the rear luggage compartment of the vehicle in order to take advantage of existing space within the vehicle.

Pressure and temperature sensors (not shown) for air, water and natural gas are mounted on the engine to regulate the pressure and temperature in response to signals generated by the electronic control means 85.

The sensors are mounted on the intake manifold, the compressed natural gas pressure regulator and the cylinder head.

A further sensor (not shown) is installed on the beginning of the exhaust pipe (not shown) to provide feedback in real time to the electronic control means 85 about the quality of the natural gas that enters the combustion chamber of the engine.

In particular, the sensor detects the amount of 02 in the natural gas and the electronic control means adjusts the parameters of the mixture of the air/natural gas supplied from the intake manifold 40 in real time to ensure the best combustion and the least amount of pollution possible.

The electronic control means 85 is programmed with operating software to ensure that the mixture of air/natural gas drawn into the combustion chamber will result in exhaust gases which meet the Euro 3 limit. The operating software may include optimum operating parameters for the vehicle in which the engine is being mounted.

The further sensor adjusts the amount of 02 in the gas and the electronic control means adjusts the parameters automatically in response.

The electronic control means 85 further ensures that the same level of air/natural gas mixture is injected through the inlet port into the combustion chamber at every intake stroke of the engine.

The electronic control means also ensures that the engine is self-regulating with regard to the natural gas composition.

Tests on an engine modified in accordance with the present invention have recorded a major reduction in emissions from the engine. The following table shows the emissions measured from a diesel engine taken from a FAIRWAY taxi cab prior to and after modification together with the Euro 3 limits for each emission.

Table 1

Values in g/km -CO NOx HC+NO PM1O x EURO 3 Limits -0.950 0.780 0.860 0.100 Diesel engine 1.835 6.574 0.555 Modified engine 0.022 0.537 0.769 0.014 These figures clearly represent a significant reduction in the emissions from an engine modified in accordance with the present invention as compared to the original diesel engine.

It will be readily apparent to a person skilled in the art that the above described transformation of a diesel powered engine to a natural gas powered engine results in an engine which produces less polluting emissions and runs more quietly than the unmodified engine.

Tests have shown that the modifications also lead to a reduction in the cost of running a vehicle. In a taxicab of the type in which the Nissan 2.7 TD diesel engine is standard, the cost savings from modifying the engine as described are expected to be of the order of around 30% -35%.

Modifying the engine to operate with a natural gas fuel supply has additional benefits to the operation of the engine including improved anti knocking potential and therefore higher efficiency of the engine.

By providing mechanical injections of the natural gas into the combustion chambers of the vehicle, the original diesel powered engine is transformed into a 4 stroke Otto combustion engine with indirect natural gas injection on the inlet manifold.

Claims

1. A method of modifying a diesel engine to operate on natural gas including the steps of replacing the diesel injector of the engine with a natural gas injector and mounting a spark plug within an existing diesel pre-combustion chamber in the cylinder head of the engine, the spark plug igniting the natural gas fuel within a specific combustion chamber in the engine.

2. A method according to claim 1, wherein the spark plug is mounted laterally with respect to a cylinder of the engine.

3. A method according to claim 2, wherein the spark plug is mounted within the existing diesel pre-combustion chamber such that the angle of the spark plug to the vertical cylinder axis is about 25 to 300. * . * . * * **

4. A method according to claim 3, wherein the angle of the spark plug :.. to the cylinder axis is 27 . * 20

*

5. A method according to any of the preceding claims, wherein the natural gas injector is installed close to the intake valve of the cylinders.

6. A method according to any of the preceding claims, wherein the method further includes the step of mounting a sensor in the exhaust manifold of the engine to monitor the quality of the gas to be provided to the combustion chamber.

I

7. A method according to any of the preceding claims, wherein the spark plug is mounted within the existing diesel pre-combustiOn chamber such that the vertical plane angle of the spark plug to the longitudinal axis of the engine is about 65 to 75 .

8. A method according to claim 7, wherein the vertical plane angle of the spark plug to the longitudinal axis of the engine is 700.

9. A method according to any of the preceding claims, wherein the io method turther includes the step of replacing the pistons of the engine with pistons which are provided with an eccentric well on an upper surface thereof.

:.

10. A method according to claim 9, wherein the method further comprises the step of aligning the spark plug with the well in the upper surface of the piston. * S * SS * SS

11. A method substantially as hereinbefore described. *, S.

12. An engine modified in accordance with a method according to any *** of claims 1-11 in which a spark plug is mounted within the existing cavity formed as a diesel pre-combustion chamber.

13. An engine substantially as hereinbefore described with reference to and as shown in Figures 1-13 of the drawings.