|MUSIC engine on the test bed.|
Recent testing of a 2.0-liter, four-cylinder implementation of the Merritt Unthrottled Spark Ignition Combustion (MUSIC) engine, initially developed at Coventry University, (earlier post) showed a 19.8% improvement in fuel economy compared to a baseline Ford 2.0L Duratec.
MUSIC is an un-throttled, high thermal efficiency, lean-burn, spark ignition system that uses an indirect combustion chamber to produce charge stratification by means of controlled air management.
|Side view of the MUSIC cylinder head. Click to enlarge.|
During the compression stroke, the piston forces air to spin around the periphery of an external cylindrical combustion chamber with a strong forward bias towards the far end of the chamber where the spark plug is situated, thereby creating a helical swirl motion. This helical swirl motion has the effect of stacking layers of rotating gas so that air delivered early in the compression stroke is situated near the spark plug end of the combustion chamber and remains in this location throughout the compression stroke.
The external chamber is connected to the combustion cylinder by an orifice large enough to minimize the pumping losses associated with the gas transfer. The orifice forces air to emerge as a powerful jet as it enters the combustion chamber. The velocity, density and temperature of the air in the jet all increase during the compression stroke. A gasoline direct injection (GDI) fuel injector delivers gasoline, or other fuels, directly into the air jet emerging from the orifice.
|MUSIC system schematic. Click to enlarge.||Cylinder head design layout and prototype for the four-cylinder engine. Click to enlarge.|
Fuel delivered early on in the compression stroke finds its way to the spark plug end and remains there, as an ignitable mixture, ensuring spark ignition, even when air with no fuel follows after the fuel injection stops. The formation of a stratified charge keeps the fuel/air mixture separate from air with no fuel in this way, while ensuring a very rapid vaporization of injected fuel. This allows MUSIC to operate completely unthrottled from idling to full load.
Timing of the beginning of fuel injection controls the local mixture strength in the cylinder, enabling exact control over local mixture strength by the formation of a pre-vaporized stoichiometric air/fuel mixture situated near the spark plug irrespective of engine speed or load. It also enables the ignition of lean mixtures which are formed later on during the fuel injection process.
The air mass flow through the orifice varies considerably during the compression stroke, increasing towards the end of the stroke. The GDI fuel injector, which can deliver at a constant rate, can form a stoichiometric mixture if it injects fuel at the crank angle position when the air mass flow rate through the orifice is some 14.5 times greater than the fuel mass flow rate through the injector.
This mixture—the Primary Mixture—will be stratified near the spark plug and there readily ignite when the plug energizes. If fuel injection continues for a longer duration after the formation of the Primary Mixture, a lean fuel/air mixture will be formed. This will be ignited by the flame produced by the combustion of the adjacent Primary Mixture. The timing of fuel injection start needed to form the Primary Mixture is advanced with engine speed to ensure that a stoichiometric mixture is formed, now at an earlier crank-angle position, as the air mass flow rate in the orifice increases with engine speed.
To achieve high loads, MUSIC needs to deliver fuel to the air jet until the end of the compression stroke when the air mass flow is very rapid and time is short. A second fuel injector—the Power Injector—is used for this with a larger fuel flow rate capacity.
The Power Injector injects axially into the air jet when the air is hot and dense, so enabling rapid vaporization over the available short time period. The Power Injector can also deliver some fuel to the air contained in the bump clearance above the piston at top dead center at the end of the compression stroke. It may be possible to incorporate the functions of the two injectors into one complex injector but the cost and complexity of such an injector may exceed the cost of two simple solenoid injectors that are currently mass-produced, according to MUSI Engines Ltd.
Unlike gasoline direct injection (GDI) engines, which rely on fuel being delivered to the spark plug at the very end of the compression stroke, when most of this fuel remains as a suspension of liquid droplets, MUSIC delivers to the spark plug a homogeneous air/fuel mixture produced at the start of the compression stroke and stratified at the spark plug end, enabling rapid ignition at all engine speeds, according to the inventor, Dr. Dan Merritt.
Implementing MUSIC requires a modified cylinder head and a direct injection fuel system. The indirect combustion chamber features an inbuilt helical swirl that can not only run successfully at air/fuel ratios of more than 100:1 but also reduces HC and NOx emissions significantly. The developers earlier estimated that up to 80% reduction in NOx and HC is possible. Load and speed control is achieved by the precise control of injection timing and duration.
|Results taken on the first test run of the MUSIC Engine constructed by Powertrain Technologies. Click to enlarge.|
The four-cylinder prototype is based on a cylinder head mounted on a Ford Duratec crankcase. The MUSIC system does not require any new supporting technology. Apart from the MUSIC cylinder head designed to promote the new combustion system invented by Dr. Merritt, the prototype engine uses currently available production components throughout.
To fit the MUSIC cylinder head, the design team at Powertrain Technologies Ltd had to work around existing fixed features such as the head bolts and the coolant and oil transfer passages. To make the 4-cylinder as versatile as possible, the key design features—the helix that generates the air motion; the transfer port plus the spark plug; and the injector locations—had to all be removable for revised designs.
Powertrain Technologies Ltd developed the test engine with the aid of a grant from the UK’s Energy Saving Trust. The 20% improvement was calculated over a number of test points in the NEDC urban cycle. The thermal efficiency increases as the engine load decreases and at near idling condition the fuel saving measured was an 42.5%.
Andrew Barnes, the Managing Director of Powertrain Technologies, expects that, in view of these results obtained at such an early stage of development, up to 25% improvement in fuel economy can be achieved in the near future.
Due to difficulties in obtaining suitable injection equipment we were unable to optimize the engine at certain test conditions, we are confident that there are a few more percentage points available when optimized.—Andrew Barnes