Challenge
A number of challenges must be overcome in the development of future fuel systems for small engines.
The desired factors are:
- Low cost fuel system
- Accurate fuel flow-rate control
- Sequentially timed with engine
- Low fuel pressure supply
- Low overall power requirement
- Small size
- Simple control systems
- Wide engine range
Up Until Now
The conventional approach used by all automotive fuel injection systems to control the quantity of fuel injected per engine cycle is the Pulse Width Modulation (PWM) concept. With this process the fuel quantity delivered is controlled by a known fluid flow rate through a fixed orifice over an accurately controlled time period.
To achieve this accurately controlled fluid flow rate, it is necessary to have a precisely controlled pressure difference across the orifice, which is usually achieved by the combination of a high pressure fuel pump and a pressure regulator with a pressure compensation feed connected to the intake manifold of the engine in the region of the fuel injector.
Along with these pressure system components it is also necessary to have an on/off flow control valve that will open and close very rapidly with high repeatability matched to a very sophisticated electronic controller allowing for precisely controlled opening periods of the flow valve.
Scion-Sprays' solution: Pulse Count Injection
As an alternative to Pulse Width Modulation, the novel concept of Pulse Count Injection (PCI) technology has been developed by Scion-Sprays to deliver the precisely controlled fuel quantity required per engine cycle.
PCI uses a small geometrically fixed volume to repeatedly inject a known amount of fuel into the engine intake manifold.
The number of pulses of injected fuel determines the amount of fuel delivered to the engine which in turn is governed by the load.
In order to achieve this fuel flow control process the injector is constructed as a simple positive displacement pump with a solenoid driven piston and cylinder working as the fixed volume displacement unit and two one-way check valves to ensure the correct flow path of the fluid into and out of the injector.
In this arrangement the single injector acts both as the pumping unit and the flow metering unit together.
Also because the flow volume delivered by each pulse is a fixed geometric volume the process is independent of differential pressure across the injector, making it very insensitive to pressure fluctuations in the intake manifold.
The PCI Technology therefore contains significantly fewer parts than the PWM system but still delivers an accurately controlled volume of fuel to the engine each cycle.
Refinement of fuel quantity control becomes therefore a matter of optimising:
- the size of the geometrically fixed volume,
- the frequency of operation of the PCI injector,
- the number of pulses required per engine cycle.
A typical fixed volume is approximately 0.5µl, with a typical operational frequency of greater than 1kHz. This range is suitable for many engine capacities in the small engine market.
Video
Please view the video in “Resources” to see an 80cc 4-stroke engine/generator running with an early development Pulse Count Injector.
The video shows an example of the easy start capability of the PCI technology and its rapid response to high load changes applied (via the electrical load) to the engine.
The laptop screen to the right of the engine cabinet shows a live trace of the PCI pulses being delivered to the engine during each engine cycle.
Summary
The Pulse Count Injector has been able to pump and spray fuel at a pulse frequency up to 1kHz.
Pulse Count Injection Technology has proven itself as a viable alternative to either standard automotive style fuel injection or carburettors.
Improvements in emissions by Pulse Count Injection fuel control alone has shown a lowering of emissions by 35% for HC+NOx and over 88% CO compared to the standard carburettor fitted engine.
The simplicity of design, control and packaging of the Pulse Count Injection solution lends itself to its use on small engines in this very cost sensitive market.