2014-32-0040 - An Advanced Fuel Supply Unit for Single Cylinder Gas Engines

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An Advanced Fuel Supply Unit for Single Cylinder Gas Engines

2014-32-0040 20149040 Published 11/11/2014

John Walters and Francois Brun Synerject LLC

CITATION: Walters, J. and Brun, F., "An Advanced Fuel Supply Unit for Single Cylinder Gas Engines," SAE Technical Paper 2014-32-0040, 2014, doi:10.4271/2014-32-0040. Copyright © 2014 SAE International and Copyright © 2014 SAE Japan

Abstract Stringent emissions legislation is being applied to small motorcycles and scooters around the world. This is forcing, gradually, the replacement of carburetors by electronic fuel injection (EFI) systems. The integration of this new technology creates new constraints on the engine and also on the vehicle. This study will provide an overview of these constraints and also technical solutions to reduce the impact on engine and vehicle. A special focus will be done on the fuel system, where the development of an advanced technology will be discussed in detail. This technology marks a break with the standard automotive fuel system architecture in order to fulfill the specific requirements of scooters and small motorcycles: low size, low weight, low energy demand, as well as simple integration. The discussion will disclose: the advantages and drawbacks of different fuel system architectures, the detailed description of the technology selected to achieve the requirements, the modelling approach used for the sizing and optimization of the design, and finally the performance achieved on the test bench.

Figures 1 and 2 are also showing that Recreation and Marine markets are mature regarding EFI technology. They re-use automotive technology. Conversely, Utility and 2/3W (Lower Cost Two and Three Wheeler vehicles) are only starting now to be equipped with EFI and will require specific solutions to achieve low cost objectives. In order to try to respect Euro3 legislations and equivalents, some countries have authorized for the small gasoline engine 2Ws, the use of carburetors with additional features controlled by a simple electronic control unit. The goal is to reduce the air/ fuel ratio variations. It is partly possible but this kind of system is quickly showing limitations and any additional performances require access to more data to calculate more accurately the set points for actuators, which have to provide more dynamic, accuracy and authority during engine operation. This is leading to the change from carburetor to EFI.

Background of Carb vs EFI and Market View The carburetor is still dominating the WW (World Wide) market of 2Ws (Two-Wheelers) motorized with a gasoline engine. The future EU4 and equivalent emission standards, which will be applied most probably in Asia in 2016-2017, will force the use of EFI systems to control small engine combustion systems and catalyst efficiency. This technology is well known and has been inherited from automotive. During the last 15 years it has been progressively introduced in the non-automotive industry, on high end products like medium to large engine displacements for motorcycles, outboard engines, ATV/UTV, snowmobiles, power supply generators, and fork lift trucks (see Figure 1). EFI can also be found on lower-cost products in mass production like scooters and small motorcycles, mainly in Europe, Taiwan, Japan, and NAFTA (see Figure 2).

Figure 1. Carburetor vs EFI trends

Figure 2. Carburetor vs EFI trends

EFI is the only technology available which is able to support real reductions of emissions thanks to its ability to control accurately the air/fuel ratio requested to maximize the Three Way Catalyst (TWC) conversion efficiency all along the vehicle life. In addition to its ability to manage accurately the air/fuel ratio, EFI has also an accurate control of all the other key parameters of the combustion system like air and spark advance. Figure 3 is showing a typical system definition of an EFI. As a result, engine performances are optimized and allow best results regarding start-ability, low fuel consumption, optimum engine performance and responsiveness, and perfect torque controllability for best drivability.

Figure 4. Simplified Air Management System

The fuel system architecture is another important functional “brick” of the control system, and has been inspired historically by automotive. There have been efforts and research during the last 10 years to simplify it. The next sections will describe the different architectures which have been proposed to the industry so far and will disclose a new concept that targets low size, low energy, low number of parts (simplicity), ease of installation, and low cost.

Various Fuel System Architectures Three families of fuel supply systems have been used to-date on small gasoline engine for 2Ws: •

Automotive architecture

•

Pump injector

•

Digital pulse pump injector

The Automotive Architecture

Figure 3. EFI Engine Management Components

Despite all its advantages, EFI has not, to date, been able to significantly penetrate the China, ASEAN or India markets. Its high cost and apparent complexity for integration and maintenance were not compatible with the maturity of these markets. So the continuous challenge of EFI is to simplify and to be more easily integrated on engines and vehicles. Some simplification around the air management system has been already proposed and is used by the industry of small gasoline engine for 2Ws, example in Figure 4.

An electrical turbine fuel pump and its filter are located at the bottom of the fuel tank. The fuel supply unit filters, lifts and pressurizes fuel into a simple fuel line which is connected between the tank and the injector. A fuel pressure regulator is installed on the fuel line and controls the pressure via bypassing the fuel flow which is not used by the engine. •

Main characteristics: in the tank at the bottom, continuously flows 2 to 3 times the engine maximum flow request to consider variation and ageing

•

Advantages: well-known technology; good control of fuel quantity; atomization and targeting

•

Drawbacks: high power supply energy; complex, large, and expensive fuel module which must consider fuel tank diversity and shape complexity; vapor generation due to fuel pressure regulator flow recirculation inside the tank.

The Pump Injector A piston fuel pump is located on the air runner close to the engine air inlet valve. The piston pump is actuated by a solenoid during the flow delivery thanks to the control of the piston stroke phased with the engine cycle. A spring returns the piston to feed the pump. The pump incorporates a shut off valve which is spraying the fuel inside the air runner. The pump is fed by a fuel line connected to the tank via a fuel filter. A second fuel line re-circulates the vapor from the top of the pump to the tank. Reference [1] as an example of this technology. •

Main characteristics: in line, true flow control thanks to sophisticated electronics and software (SW); the quantity of fuel generated by the pump corresponds to the quantity of fuel requested by the engine

•

Advantages: compact packaging, “in-line solution” which does not require fuel tank modification; high peak current but low average energy demand.

•

Drawbacks: complex and expensive component, sensitive to fuel vapor lock, return line to the tank is requested, limited performance for fuel atomization and targeting; installation on engine has to respect strict rules

Figure 5. FSU fuel system components

Referencing Figure 6, the FSU operates by drawing fuel into the pumping chamber with a solenoid actuating a plunger, as commanded by the ECU, and then discharging pressurized fuel into the delivery line with a spring, in synchronization with the engine speed. The spring is set to provide the required fuel delivery pressure, and check valves control the intake and outflow from the pumping chamber. The amount of fuel taken by the injector will determine the extension of the spring, and so will be delivered automatically with no intervention of the control system, unlike the system described in [1].

The Digital Pulse Pump Injector The digital pulse pump is a variant of the pump injector where the size of the device is greatly reduced, thereby reducing the cost. Each pulse corresponds to a low and precise amount of fuel flow. For a given fuel flow request, the pump is actuated multiple times during engine air inlet stroke. Reference [2] as an example of this technology. •

Main characteristics: in-line, true flow control thanks to sophisticated electronics and SW, the quantity of fuel generated by the pump corresponds to the quantity of fuel requested by the engine

•

Advantages: very compact packaging, “in-line solution” which does not require fuel tank modification; medium peak current but low average energy demand; low cost.

•

Drawbacks: volumetric efficiency sensitive to internal and external parameters, sensitive to fuel vapor lock, return line to the tank is requested, reduced performances for fuel atomization and targeting, injection phasing limited, dynamic flow range limited, installation on engine has to respect strict rules

Considering the different advantages and drawback of these different fuel systems, SYNERJECT has developed a new concept fuel supply unit (FSU) with the intention to keep all the advantages without the drawbacks.

Advanced Fuel Supply Unit (FSU) Basic Fuel System and FSU Operation A novel fuel system architecture is proposed that can serve to meet these market requirements. This system (Figure 5) contains a standard automotive-style fuel injector, an Engine control unit (ECU), and an FSU which provides nearly constant fuel pressure output.

Figure 6. FSU Construction

The average delivered pressure (Pavg) will decrease with injector demand (per cycle) due to the change in spring force applied through the plunger delivery motion. The trend of lower pressure at higher injector flow demand can be seen in Figure 7, and is directly related to the spring stiffness. Additionally, the non-ideal behavior of the check valves begins to significantly affect the pressure/flow behavior of the FSU at speeds above 7200RPM, as time to complete the inlet and outlet flow is shorter and begins to affect volumetric efficiency. This effect can be seen in comparing the low speed and high speed characteristics in Figure 8. This pressure/flow characteristic is acceptable can be compensated by the control system.

tdelivery: the time required for discharge of the pressurized fuel into the downstream line. The spring force is applied to the plunger to pressurize the fuel. The summation of the fundamental time durations td, ts, tf, tr, and tdelivery will govern the maximum operating speed. Considering that the target maximum speed is >10,000 RPM, this fundamental cycle must be accomplished in less than 11ms, even considering worst case temperature, speed, and flow per cycle. FSU sizing and analyses provided time duration targets which have driven the required performances and design choices of sub-components such as valves and the solenoid. Reference Table 1. Table 1. Target durations of key steps in fundamental cycle and critical subcomponents Figure 7. Pressure vs Injector Demand Characteristic

We can better understand the critical workings of the FSU operation by closely examining one cycle and the fundamental steps inside each cycle. Figure 8 helps to explain the sequence of these steps.

Operational and Functional Benefits Simplification

Figure 8. Fundamental cycle

td: the time where current is rising with no plunger motion, beginning with start of current and ending when the magnetic force overcomes the spring preload. ts: time duration of the plunger suction motion, where the plunger is pulling fuel into the chamber and attains full lift. tf: once the plunger reaches full lift, some additional time is required to completely fill the chamber and stabilize the inlet check valve. tr: after the completion of the command pulse, time is required for magnetic field breakdown resulting in only the spring force being applied onto the plunger.

The FSU is providing controlled fuel pressure over all engine speeds and injector demand. Therefore a pressure regulator is not required. An automotive-style injector can be installed, calibrated, and controlled in the same manner as existing port injection fuel systems. Re-using the automotive-style injector allows continued capitalization of the advances brought by this industry in spray development and component cost. The FSU brings low size, low energy, and easy implementation and installation (no changes to vehicle architecture required) for fuel delivery. The axial plunger-in-bushing motion (high L/D, very low side forces) essentially eliminates subcomponent wear over time. The FSU will not produce the contamination generated by conventional fuel pumps powered by brushed DC motors, which typically require a downstream filter required to protect the fuel injector. The low weight and small size of the FSU allows simple packaging in the target market applications, and particularly lends itself well to integration with simple fuel delivery modules. Figure 9 shows an example that has been developed.

Figure 9. Simple fuel delivery module

Energy The FSU is actuated in sync with the engine speed and so the energy required to drive the device is proportional to engine speed. This results in the power demand being notably lower than conventional fuel pumps in all but the highest engine speeds. FSU average current draw vs vehicle speed is shown in Table 2, and is compared to a best-in-class conventional turbine fuel pump that is applied in similar applications. Table 2. Current Draw vs Vehicle Speed

Design Considering Boundary Conditions Some of the challenges in development were related to achieving functionality at the environmental boundary conditions. One example is the requirement to operate nominally at high engine speeds at maximum ambient temperature. Without proper consideration in the solenoid design, elevated ambient temperature and component selfheating (at high frequencies / high duty cycle) can significantly decrease the solenoid force. This is due to the resulting higher resistance in the coil leading to lower current, hence lower magnetic force, causing unacceptably long actuation times (mainly td - Ref Figure 8). To design a solenoid which achieves the high speed / high temperature operating requirement, consideration of all performance parameters is necessary, due to the interactions between coil design, magnetic circuit design, operating speed, power input, and ambient temperature - Figure 10. With strong constraints for current supply and environmental temperature, and mandatory speed requirement (achievable by reaching target solenoid force at td), the most effective area of the design on which to act was the magnetic circuit design.

Figure 10. Solenoid design interactions

The efficiency of the solenoid's magnetic circuit to generate force from input current is typically expressed in terms of Force/ Magnetomotive force (mmf), where mmf = Amps × coil turns. Defining the target mmf was an iterative process due to the interactions of physical phenomenon in Figure 10, but we can illustrate the method via the final design values. With final coil design parameters, Power input, and heat transfer coefficient from the coil, Tcoil and the current available at td (at the maximum environment temperature), target mmf can be calculated. There is 2.5A available (at td = 2ms) with 220 coil-turns = 550mmf, which must be sufficient to create our target force. Figure 11 shows an example of initial design solenoids where 800+ mmf was required to reach the target force. With this level of efficiency, low current (high temperature) operation would not be achieved. To meet the target mmf, the cross section area of the magnetic components were increased (in critical areas) and air gaps were decreased, which lowered the overall reluctance. These are the normal methods for improving magnetic efficiency. The real benefit during development was the clear and accurate setting of the target required to achieve the performance, and the ability, via simulation, to simultaneously consider all of the other linked parameters. Figure 12 highlights the increase in magnetic efficiency that was achieved during development. The capability to produce force at relatively low current levels greatly enhances the performance at max boundary temperature. The target force is achieved prior to the target td, leading to fully capable speed performance over the expected thermal operating envelope.

require specific solutions, and the FSU has been developed as a key component of a novel solution. The proper selection of architecture, the integration of multiple functions in one packaging, and size reduction are the keys drivers to optimize a system.

Figure 11. Improvements to Mag Circuit - Improved capability to produce Target Force at lower mmf

Designing a pump that is actuated every engine cycle requires complex design optimization involving simultaneous physical phenomena. Thanks to these efforts, the FSU is able to achieve the lowest cost, size, weight and energy demand requested for low displacement single cylinder engines equipped with EFI.

References 1. Ishibe, E., Torii, K., and Kasai, T., “Development of a 4-Stroke Small-Displacement Scooter with Discharge Pump Type Fuel Injection System,” 2005. 2. Allen, J. and Ravenhill, P., “A Novel Low Cost High Frequency Fuel Injection System for Small Engines,” SAE Technical Paper 2006-32-0107, 2006, doi:10.4271/200632-0107. 3. Chai, Hi-Dong. “Electromechanical Motion Devices.” Upper Saddle River, NJ: Prentice Hall PTR, 1998. Print.

Figure 12. Dynamic Force Improvements

Conclusion The high volume / low displacement gasoline engines applied in the 2W and Utility markets are undergoing a massive move from carburetor to EFI. New specific components are requested because vehicle size and cost, mission profile and boundary conditions in use are very different from automotive. The fuel system is one of the engine sub-systems which

Definitions/Abbreviations 2W - Two Wheeler (Vehicles) 2/3W - Two or Three Wheeler (Vehicles) ECU - Electronic Control Unit EFI - Electronic Fuel Injection FSU - Fuel Supply Unit SW - Software WW - Worldwide

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