The operation of internal combustion engines is known to be inefficient in various ways, both from the standpoint of fuel combustion and wear on the engine parts. The improvement of the operation of combustion engines in order to use fuel more efficiently and protect the environment is the subject of dedicated research and development all over the world.

Some efforts have concentrated on the ignition circuit itself, and in particular the shape of the spark. In a conventional ignition circuit the current versus time relationship begins with a sharp increase in current when the spark first occurs, followed by a period of lower current flow before voltage across the spark gap decays.

In WO-92/08048, it is postulated that the spark itself should be prolonged in order to achieve combustion, the period of lower current flow following the initial discharge being described as relatively ineffective. It is suggested that a capacitor positioned in the ignition circuit will be effective to maintain the sparks for longer and improve the combustion. A diode is also positioned in series with the capacitor. In fact, commonly available diodes and capacitors are not suitable to withstand the high temperatures and voltages present in combustion engines and until the present invention it is believed that no such components were available.

WO-94/17302, by the same inventor, suggests the use of relatively high value capacitors to achieve the desired effect.

This invention is based on the realisation that it is the low current low voltage period following the initial discharge, previously described as ineffective, which is essential for maintaining combustion.

According to the present invention, it is not the initial high voltage, high current spark which is prolonged, but the subsequent lower voltage lower current period sometimes referred to as the"back porch"of the spark characteristic.

The present invention provides a spark ignition circuit comprising a source of high voltage and a spark gap including a diode and a capacitor connected in parallel between the voltage source and the spark gap.

The diode may be arranged such that when an arc is deliberately induced across the spark gap, the diode conducts the arc current. In an effect to be described in more detail below, the diode/capacitor combination reduces opposite polarity voltage spikes following an initial discharge and the capacitor serves to maintain a low voltage across the spark plug terminals for a longer duration than was previously achievable.

Since a capacitor/diode combination can be installed in any existing engine, the invention also encompasses the use of a capacitor and a diode in parallel in an ignition circuit between the high voltage source and the spark gap.

Certain types of diode have an inherent junction capacitance such than when reverse biassed, they are equivalent to an open circuit and a capacitor in parallel. Therefore, another aspect of the invention provides a spark ignition circuit comprising a high voltage source and a spark gap and a diode between the voltage source and the spark gap, the diode having an inherent capacitance such that it acts as a capacitor when reverse biassed.

One of the most important effects of the diode seems to be its ability to"shorten"any opposite polarity voltage spikes following an initial discharge so that they do not interrupt the combustion as presently seems to occur with conventional internal combustion engines. Thus the diode is preferably a fast acting diode with a short reverse recovery time. The presently preferred diode has a reverse recovery time of typically 200ns when switched from 100mA at a rate of-200 mA/. s under a reverse voltage of 100 volts or more. A reverse recovery time of the order of 200ns is therefore desirable.

In contrast to the arrangements disclosed in WO-94/17302, a relatively small capacitance, or junction capacitance, is desirable, typically no more than lpF. The presently preferred diode has a junction capacitance of no more than lpF, preferably 0.1-lpF. The appropriate value may depend on the applied voltage.

The diode should be able to withstand the very high voltages present in ignition circuits. Preferably the diode can withstand a reverse breakdown voltage of 24,000 volts or more, and can preferably withstand forward voltages of the same order. In the preferred embodiment of the invention the diode can withstand forward and reverse voltages of between 24,000 and 90,000 volts.

The voltage source may be a pulse transformer or coil.

An embodiment of the invention will now be described by way of example only and with reference to the accompanying drawings in which: FIGURE 1 shows a typical ignition circuit with an ultra fast soft recovery diode positioned between the spark plug terminals and the transformer; FIGURE 2 is a circuit diagram similar to Figure 1 showing the equivalent circuit of the diode; FIGURE 3 is a diagram showing the variation of voltage with time during a typical ignition cycle when no diode is present in the ignition circuit; FIGURE 4 is a diagram showing the variation of voltage with time using the circuit of Figure 1; FIGURE 5 is a diagram showing a suitable diode potted and provided with suitable connectors for use in an internal combustion engine; FIGURE 6 is a schematic diagram of a typical coil and battery ignition circuit, according to the invention; FIGURE 7 is a schematic diagram of a DIS ignition circuit according to the invention; FIGURE 8 is a schematic diagram of a CDI inverter ignition system according to the invention; and FIGURE 9 is a schematic diagram of a ballasted ignition circuit according to the invention.

The operation of spark ignition systems is well known in the art and will not be described in detail herein. Figures 1 and 2 show the usual pulse transformer 10 whose primary winding 11 is connected to a pulsed current source (not shown) including a battery. The transformer secondary winding 12 is connected to a spark plug 13. (Usually the connection is intermittent, and is governed by a distributor, omitted from Figures 1 and 2 for the sake of clarity.) A diode 15 is connected between the transformer secondary winding and the spark plug 13.

In order to explain the operation of a spark ignition circuit according to the invention, the stages in conventional spark ignition will first be briefly explained with reference to Figure 3. Figure 3 shows the voltage characteristics of a conventional spark ignition circuit, similar to that of Figure 1 but omitting the diode 15.

Current is supplied to the transformer primary winding 11 for a predetermined period (the dwell period) which ends at time t,. The current sets up a magnetic field and as soon as the current supply is switched off the magnetic field decays inducing a voltage in the primary and secondary coil windings. The voltage induced in the secondary winding 12 is routed to the spark plug (via the distributor). The voltage, which is negative, increases in magnitude rapidly until an arc is created across the spark plug gap at time t2.

Current then flows and the voltage decays, first rapidly and then more slowly, during time t3 to t4. The current sets up a reverse polarity magnetic field in the transformer secondary winding 12. The voltage eventually decays to zero at time t5, at which time current ceases to flow and the reverse effect occurs, with a smaller opposite polarity voltage spike occurring at time t6. The voltage decays following a damped sinusoidal waveform until the dwell period recommences and the cycle is repeated.

The time t3 to tS, sometimes referred to as the"back porch"has been found to be particularly important for maintaining combustion in the cylinders and ensuring maximum use of fuel, and hence efficiency. Any additional voltage spikes after the back porch time has ended are unwanted and in fact cause wear on the spark plugs.

The operation of the circuit of the present invention will now be described with reference to Figures 2 and 4. From time t, to t6, the voltage across the spark plug varies in the same manner as in a conventional spark ignition circuit. From t, to t the diode 15 is forward biassed and current flows in the direction of the arrows in Figure 1.

The particular diode used in the circuit is equivalent to the circuit shown in dotted lines in Figure 2, namely a diode D 1 having a simple one-way flow path, in parallel with a capacitance, known as the junction capacitance, CJ, and in series with a resistance R1.

After time t6, the following effect is believed to occur: When a reverse polarity voltage spike occurs at time t6, the diode D 1 is reverse biassed and the junction capacitor CJ begins to charge. Due to the reverse recovery characteristics of the diode, a small reverse current flows causing the positive voltage spike at t6 to decay more quickly than if the diode was not present. The negative voltage present after time t6 is the voltage accumulated across the capacitor CJ which decays according to the time constant of the capacitor CJ. With a suitable choice of diode the negative voltage after time t6 can be approximately equal to the voltage present during time t3 to t,.

The use of a diode which acts as a capacitor when reverse biassed has been found to significantly improve the fuel combustion in internal combustion engines. It is believed that the positive voltage spike is so short and sharp that it is imperceptible and the effect of the diode is simply to extend the"back porch"of the voltage characteristic. The continuing voltage which is present after the initial spark is known to be important for maintaining combustion.

Also, more energy is present due to the concentration of the normally alternating current being concentrated into a longer time due to the extension of the back porch.

The effects described above have been achieved using as the diode 15 a high voltage fast soft-recovery diode available from Philips'semiconductors.

Types BY7I4 and BY 8424 have been found to be suitable although BY8424 is preferred. These diodes are designed for use in television circuits and diodes of this type have apparently not been used in ignition circuits.

The diode is potted in a suitable dielectric potting compound to form a cylinder of approximately 1.5 cm diameter and 5 cm long as shown in cross section in Figure 5. Suitable terminals are added to the cylinder. Simply placing the diode 15 in circuit with no potting compound or significantly less compound would result in sparks being produced across the diode. The potting compound must obviously be suitable to withstand the high temperatures in internal combustion engines. Specifically, for use in cars it must be a"car grade"material meting standard I EC 250. One suitable material is "Formulation 600"Epoxy Synthetic Polymer although other equally suitable materials are available in the electronics industry. The finished unit shown in Figure 5 has no mechanical parts such as screws or nuts and bolts. The ends are preferably nickel chromium (as used in conventional spark plug ends). The unit is completely sealed with no moving parts, and has an electric strength of 140 kV/cm. In the illustration of Figure 5, the cylinder has a reduced cross- section central barrel allowing faster cooling during operation and a reduced weight loading for the king lead.

The diode can be installed in any existing ignition circuit with no other modifications being required. Alternatively it can be installed in new ignition circuits. The diode can be installed in any location in the circuit from inside the coil to inside the spark plug.

Figures 6 to 9 show a few examples of ignition circuits in which the unit of Figure 5, designated GB60 is installed. In all cases the unit is installed between the ignition coil secondary winding and the spark gap.

The use of the diode has been found to achieve virtually complete combustion of fuel in vehicles equipped with catalytic converters (zero CO and HC). The following two sheets show, by way of example, the results of tests on two vehicle engines.

Where catalysts cannot be fitted or actually manufactured for vehicles or engines, the use of the diode achieves typically up to a 90% reduction in the CO exhaust emissions and typically up to a 70% reduction in hydrocarbon emissions.

Figures will vary according to the pulse transformer or coil fitted and the fuel used and the type of carburettor installed in the normally aspirated engine.

Complete combustion of fuel has enormous benefits including the following: Greater energy in the spark does help create improved combustion.

The improved combustion alters the burning bar pressure of the fuel.

The improved combustion and burning bar pressure increase the compression ratio per cylinder to near maximum i. e. 100% of original design of operating CR.

The increased compression and combustion reduce exhaust emissions and further improve the fuel economy.

Complete combustion improves the vehicle's responsiveness.

The total burn of hydrocarbons keeps the oil and filters clean for far longer than in a conventional engine environment.

The total burn of carbon monoxide and hydrocarbons in an engine maintains clean spark plugs, valve seats and exhaust systems.

A clean exhaust system helps to maintain a clean and more efficient Auto Catalytic Converter.

As the AFR is leaner at a high rpm and the operating temperature of the engine is kept to a minimum and due to the complete combustion of the carbon monoxide and hydrocarbons the NOx emissions are reduced.

Water content and (O) oxygen content of the exhaust emissions increase.

As the more complete combustion of the fuel occurs more power is generated even in the lean AFR high rpm environment and the CO2 is slightly reduced or remains constant.

Improved combustion of the fuel within the dwell time also helps to eliminate hot spots within the combustion chamber.

The increased compression and burning bar pressure create greater MPG without having to alter the engine.

Results from independent test houses measuring the effects of the use of the Philips diode BY8424 on vehicles show that zero CO and hydrocarbon emissions are achievable on catalyst installed vehicles and are greatly reduced on normally aspirated vehicles. Both types of vehicle show lower CO2 and NO : c <BR> <BR> emissions, whereas the H20 and O or O 2 content of the emissions is increased.

Cavalier1800SRI GB60 EMISSION RESULTS BASED ON BEFORE AND AFTER FITTING BeforeGB60AfterGB60Cavalier1800SRI CO2 0.25% -97.5%%Change Cavalier1800SRI BeforeGB80AfterGB60#avalier1800SRI HC (ppm) 5cO 35 %Change-93.8% i Cavalier1800SRI BeforeGB60AfterGB60Cavalier1800SRI CO 0.03% %-97.8% Cavalier1800SRI CavalierGB60AfterGB60Before 02 4. SO% 20. 90% %Change 335. 4% Prepared by 03/07/97International Rover 827i, 16V 1996WithCatalyst, GB60 EMISSION RESULTS BASED ON BEFORE AND AFTER FITTING Rover GB60AfterGB60Eefore CO 0.00% -100.0%%Change CORover827i Rover GB60AfterGB60Before HC (ppm) 38 9 % Charge-78. 3,"a HC(ppm)Rover827i Rover827i AfterGB60BeforeGB60 CO2 15.30% 0.9%%Change CO2Rover827i BeforeGB60AfterGB60Rover827i 02 0.00% 0. 00% %Change0.0% O2Rover827i FXInternational03/07/97Preparedby