FIELD OF THE INVENTION
This invention relates to an internal combustion engine suitable for small engine operation using lean fuel/air mixtures.
BACKGROUND OF THE INVENTION
Internal combustion engines are known in which each cylinder is provided with two valves, namely inlet valve and exhaust valve. These valves open in a cavity in the cylinder head conventionally known as a combustion chamber. Typically, such engines are provided with only one spark plug per cylinder. One of the disadvantages of such an arrangement is that the process of combustion from spark initiation to completion takes a comparatively long time. At higher engine speeds, the available combustion duration may be insufficient, resulting in incomplete combustion and consequent loss of power, fuel wastage and increase in emissions.
These problems may be addressed by employing an engine having two spark plugs. This is counter-intuitive for use in a small bore engine, where flame propagation distances are relatively small and normally sufficient to enable sufficient combustion. Space constraints also intruded.
The Applicant has developed fuel efficient engines that include a twin spark plug configuration, the configuration being implemented either by providing a spark plug extending through the timing chain cavity or distal from the timing chain cavity. The latter arrangement avoids compromise of spark plug selection, design or location by the timing chain cavity and the lubricant contained within it. However, both arrangements allow packaging of a twin spark plug system within a small bore engine where twin or dual spark plugs would not be used.
An object of the Applicant has also been to provide a small bore engine which enjoys an improvement in driveability. While improvements in fuel economy and emissions performance have been achieved with current engines, driveability has remained a focus for further improvements in performance. That is, fuel economy is an important factor but it may be achieved at the expense of driveability.
A measure of driveability is the coefficient of variance ("GOV") of engine torque within an engine cylinder. A low GOV indicates good combustion stability and

increasing GOV indicates increasing instability and poorer driveability. The Applicant has the object of providing an engine with dual spark ignition and improved driveability.
With this object in view, the present invention provides an internal combustion engine comprising:
(a) a combustion chamber for combustion of a charge and having a valve suite comprising an inlet valve and an exhaust valve; and
(b) at least two ignition means
wherein the inlet and exhaust valves are offset and the valve suite is configured to induce both tumbling and swirling motion in a charge present within the combustion chamber.
The engine of most concern to the applicant is a small bore engine, most typically a single cylinder small bore engine. Typical characteristics of a small bore, small capacity or small displacement engine for use in the practice of the invention include a swept cylinder volume ranging from 75cc to 225cc, preferably in the range 100cc to 200CC, and cylinder bore diameter 45mm to 70mm, employed as prime movers for operation of two or three wheeled vehicles or other motorized vehicles, for example, motorcycles. Such an engine is advantageously a single cylinder engine which may be a two valve overhead cam engine.
Tumble and inclined tumble ("swirl") are rotational flows, recognized in engine fluid dynamics, which cause mixing of fuel and air within an engine. Tumble is induced in the Applicant's engine by selecting an appropriate entry angle for the inlet valve particularly at the inlet port. Arrangement of airbox and the nature of fuel delivery to the engine, in the small engine, are influences on the entry angle and the engine is packaged such that the entry angle will permit formation of tumbling flow. As parallel an alignment of an axis of the inlet port relative to a longitudinal axis of the combustion chamber as may be achieved is preferred to maximize the tumbling effect. The further swirl rotational flow is induced by the offset configuration of the inlet and exhaust valves. The combination of tumbling and swirling flows in the Applicant's small bore engine, enables enhanced charge mixing and better combustion stability and driveability.

The engine is operated with exhaust gas discharge through the exhaust valve following a combustion event without exhaust gas recirculation. While controlled exhaust gas recirculation ("EGR") may enable reduced NOx emissions, through reduction of combustion chamber temperatures below the level at which NOx species form, and is conventional practice for achieving this end, EGR tends to reduce combustion rate making stable combustion more difficult to achieve. In the case of the Applicant's engine, NOx emissions are acceptable, or remediable through appropriate catalytic treatment, and the parameter to be improved is combustion stability as reflected by a lower GOV value.
As noise and harshness may also affect driver comfort, the spark plugs may be operated in accordance with the strategy described in the Applicant's co-pending POT Application No. WO 2007032020, the contents of which are hereby incorporated by reference, and in which a control unit for the engine retards or stops operation of an ignition means, or spark plug, in response to the rate of rise of combustion pressure as proxied by rate of rise of engine speed or engine acceleration, rising above an acceptable threshold value.
The engine enables improved driveability, through improved combustion stability, while maintaining the benefits of twin spark plug ignition in the small bore engine.
The internal combustion engine of the invention may be more fully understood from the following description of a preferred embodiment thereof made with reference to the accompanying drawings in which:
Figure 1 is a graph of Coefficient of Variance as a function of BMEP and engine speed;
Figure 2 is a bottom view of a cylinder of an internal combustion engine in accordance with one embodiment of the invention; and
Figure 3 is a side sectional view of the cylinder head of an internal combustion engine of Figure 2.
Figure 4 is top view of the engine cylinder depicting inlet and exhaust ports and combustion chamber.
Referring now to Figure 1, is shown a graph of Coefficient of Variance {"COV) of a small bore single cylinder internal combustion engine 100 having a twin spark plug

ignition system and a valve suite arrangement as otherwise described with reference to Figures 2 and 3. This graph shows GOV associated with good driveability during normal engine operating conditions due to the combined effects of the tumble and swirl rotational flows achieved in the combustion chamber 110 of engine 100. These flows enable thorough charge mixing, thereby promoting efficient combustion and good combustion stability.
The bottom of the cylinder head 44 of the engine 100 is shown in Figure 2. The cylinder head includes the combustion chamber 110 having an inlet valve 23 and an exhaust valve 24 selected to be of smaller size than conventional and to deliver less power but to the benefit of improved fuel economy, perhaps at least 5% over comparable vehicles fitted with a single spark plug. The exhaust valve 24 has its exhaust port 24a.
The inlet valve 23 has its entry port 23a to combustion chamber 110 arranged to induce a tumbling motion to incoming air to the combustion chamber 110 as shown in Figure 3. Tumbling airflow direction is shown by arrows in this figure. Tumbling flow is a rotational flow that enhances charge mixing, thus facilitating combustion and enhancing combustion stability as evidenced by GOV performance as illustrated in Figure 1. The inlet port 23a discharges air in a direction of a base of combustion chamber 110 and has its direction of discharge airflow approaching parallel alignment with a longitudinal axis of the combustion chamber 110. That is, inlet port 23a has a flow axis approaching parallel with longitudinal axis 110a of combustion chamber 110. The closer to parallel alignment that is achieved, the better the degree of tumbling and charge mixing that may be achieved. However, location of the engine airbox and the fuel delivery components will affect the entry angle of the inlet port 23a and a compromise may be required, particularly for the small bore engine. To that end, the engine 100 is designed to further enhance charge mixing as described below.
Dual spark plugs 40 and 42, each spark plug being of conventional manufacture, are fitted in the cylinder head 44, being located in the combustion chamber 110 and allowing the engine to be operated in a lean combustion mode to improve fuel economy. As seen in Figure 2, the combustion chamber 110 may be considered to be divided into quadrants. The inlet and exhaust valves 23 and 24 may be considered to reside in diagonally opposed quadrants. Such a valve configuration is

an "offset" configuration, normally associated with lesser power output than for "in¬line" valve configurations. The offset configuration is selected to induce a swirl motion in charge flow also enhancing charge mixing and improved combustion stability. The swirling motion in the charge flow is depicted by arrows in Figure 4. The degree of swirl achieved in the engine 100 is a function of the degree of offset of the inlet and exhaust valves 23 and 24 and the degree of offset may be selected accordingly having regard to engine packaging constraints on the degree of offset. Thus, engine 100 is designed to promote a combination of swirl and tumble flows which promote charge mixing, combustion stability and enhanced driveability as illustrated by the GOV performance as shown in Figure 1.
Spark plugs 40 and 42 reside in the remaining diagonally opposed quadrants of combustion chamber 110.
The inlet and exhaust valves 23 and 24 of engine 100 are actuated by a rocker arm assembly as described in the Applicant's co-pending International Patent Application No. WO2007080603, the contents of which are hereby incorporated herein by reference. Such a rocker arm assembly 18 is configured to provide a greater space for location of the twin spark plugs 40 and 42, this being important in small bore engine 100.
By providing the two spark plugs 40 and 42, initiation of sparks originates at diametrically opposite locations in combustion chamber 110. Accordingly, combustion may be completed in a comparatively shorter time duration leading to reduction in emissions and improved fuel consumption. The reduced combustion duration permits use of a lesser amount of fuel/air mixture or a lean mixture without affecting engine performance. Enhanced charge mixing, as achieved through the tumbling and swirling rotational flows, also enables enhanced combustion stability and driveability under normal engine operating conditions. Timing and operation of the spark plugs 40 and 42 may be controlled having regard to the charge mixing behaviour of engine 100.
As driver comfort may be affected by noise and harshness, the operation of spark plugs 40 and 42 may also be controlled by an engine control unit to minimise these effects, particularly when the engine 100 is in acceleration. At such times, the rate of rise of combustion pressure may induce a high noise level that may be controlled by

retarding spark plug operation, or stopping ignition events, when the rate of rise of combustion pressure, as proxied by rate of rise of engine speed or engine acceleration, exceeds a threshold limit. This also enhances driving comfort in a vehicle, such as a motorcycle, using the engine 100. Further description of the noise and harshness control methodology that may be employed is described in the Applicant's co-pending International Patent Application No. WO2007032020, the contents of which are hereby incorporated herein by reference.
Finally, engine 100 is operated without exhaust gas recirculation ("EGR"), Whilst controlled EGR is conventionally used to minimise NOx emissions, it may reduce combustion stability contrarily to the Applicant's object. Alternative NOx emission control strategies are therefore used in engine 100;
Modifications and variations of the internal combustion engine of the present invention will be apparent to the skilled reader of this disclosure. Such modifications and variations are deemed within the scope of the present invention.

WE CLAIM:
1. An internal combustion engine comprising:
(a) a combustion chamber for combustion of a charge and having a valve suite comprising an inlet valve and an exhaust valve; and
(b) at least two ignition means
wherein the inlet and exhaust valves are offset and the valve suite is configured to induce both tumbling and swirling motion in a charge present within the combustion chamber.
2. The engine of claim 1 being a small bore engine.
3. The engine of claim 2 being a single cylinder engine.
4. The engine of claim 2 or 3 having a swept cylinder volume from 75cc to 225cc, preferably in the range 100cc to 200cc.
5. The engine of claim 4 having a cylinder bore diameter in the range of 45mm to 70mm.
6. The engine of any one of the preceding claims wherein an entry angle for the
inlet valve is selected to induce tumbling motion.
7. The engine of claim 6 wherein the entry angle of the inlet valve is an entry
angle at an inlet port to the engine.
8. The engine of claim 6 or 7 wherein the arrangement of engine airbox and the
nature of fuel delivery are selected to achieve the selected entry angle to induce
tumbling motion.
9. The engine of claim 7 or 8 wherein alignment of an axis of the inlet port
and a longitudinal axis of the combustion chamber approach parallel.

10. The engine of claim 9 wherein alignment of said axis of the inlet port and
said longitudinal axis of the combustion chamber is substantially parallel.
11. The engine of any one of the preceding claims operated without exhaust gas
recirculation.
12. The engine of any of the preceding claims being operable in lean combustion
mode.
13. The engine of any one of the preceding claims wherein a control unit for the
engine retards or stops operation of an ignition means in response to the rate of
rise of combustion pressure in the combustion chamber.
14. The engine of claim 12 wherein the rate of rise of combustion pressure in
said combustion chamber is proxled by rate of rise of engine speed or
engine acceleration rising above an acceptable threshold value.