Fuel Cooling Needs for Advanced Diesel Engines
Michael Davies, John Burgers, and Nick Kaiman
Design changes in high pressure diesel injection pumps have increased the heat input to the fuel beyond acceptable levels. This paper describes the source of the extra heat, its ffects, and various ways of dissipating it.
With the progression of legislation (CARB, EPA, EC, etc.) to lower vehicle emissions, there is a need to further reduce tailpipe emissions from diesel powered vehicles. Diesel engines offer an advantage over their gasoline powered counterparts because of their durability, lower fuel consumption, safer less volatile fuel, and low Hydrocarbon (HC), Carbon Monoxide (CO), & Carbon Dioxide (CO2) emissions. The challenge facing the diesel industry is to lower Oxides of Nitrogen (NOx) and Particulate Matter (PM) emissions without hindering vehicle performance or efficiency.
This tightening up of allowable NOx & PM emissions is pushing conventional emissions control strategies beyond current capabilities. Development in current technology includes both in-cylinder and after treatment emissions control strategies. After treatment strategies include after burners, particulate traps and filters, close coupled catalysts, and lean NOx catalysts. Fuel additives and fuel refining chemistry changes are also beneficial in reducing emissions. In cylinder control of emissions involves controlling the location, duration, timing, and time-volume history of combustion. Physically, this translates into keeping the flame away from the quenching effect of the walls, controlling the amount of lubricating oil entering the cylinder, the placement and direction of the fuel spray and the combustion bowl geometry. Recirculating combustion gases back into the cylinder intake (Exhaust gas recirculation) also reduces emissions. This effect is enhanced by cooling the returned gases as more mass can be returned. Normally, though, EGR can only be done at part load. Electronic control of the fuel injection has greatly reduced emissions by taking advantage of variable timing, controlling the time-volume history, and utilizing the ability to map the limits of fuel injection to prevent excessive smoke generation. Breaking the injection into multiple events is also beneficial. Mostly, however, there has been a change towards direct fuel (DI) injection rather than indirect (IDI), and a trend towards higher injection pressures for better control of the spray and its duration. The high pressure fuel injection strategy greatly increases the work input into the fuel and substantially raises the fuel temperature. There are, however, various strategies for cooling the fuel to manageable temperatures, the drive cycle impact on fuel cooling needs and the causes and effects of high fuel temperature.
MOTIVATION FOR FUEL COOLING
Why Is There More Heat?
One solution to increase performance, increase mileage and reduce emissions that is showing promise is to raise the injection pressure of the fuel, in some cases to 1600 bar (23520 psi) with rotary injection or common rail pumps, and even higher to 2,000 bar (30,000) with unit injectors. This will lead to finer fuel atomization and better rate shaping. The higher pressures allow for the fuel to be injected in a more tightly controlled window of the crank angle, which optimizes the combustion event. To reach the full benefit of electronic control of fuel injection, designers of injection systems are also employing high speed solenoid valves to end the injection event. Such quick injection ending reduces unburned HC and PM formation. However, this new strategy increases the work in the spilled fuel in comparison to older technology injection pumps which incorporate an 'over the cam' approach to control the injection event. Newer electronically controlled high pressure injection systems can produce 0.15 to 3.OkW of waste energy to the recirculated fuel, versus older systems which create 0.15 to 1.8kW. This increased waste energy elevates the fuel temperature above that tolerable at the injection pump inlet.
Why is There a Temperature Limit?
There are many reasons why the fuel temperature needs to be limited:
1. Modern HSDI engines for passenger cars are designed for operation at an optimum fuel temperature of 45°C.
2. Increased temperature reduces the density of the diesel fuel. Since a constant volume of fuel is injected on each stroke, the reduced density means that a smaller mass of fuel is available to be burned, affecting the stoichiometry of combustion and also reducing engine power.
3. Increased temperature reduces the viscosity of the diesel fuel. The reduced viscosity increases the leakage flow past the pistons of the injection pump, which increases the drive power and pumped volume of the injection pump.
4. Increased temperature increases the bulk compressibility of the fuel. The pump must therefore do more work to pressurize the fuel at a higher temperature. It also affects the pressure wave dynamics.
5. A limit exists on the fuel tank temperature due to the use of plastics. As well, other material compatibility conflicts arise with the use of elastomers.
6. Fuel lubricity is reduced at elevated temperatures. This is very harmful as the fuel itself is used to lubricate the injection pump.
7. There is also incorporated integrated electronic circuit chips onto injection pumps, which must be cooled below their 120°C limit.
8. Due to the extremely precise and small volume of fuel per injection event, each injector must supply the same volume of fuel at the same temperature.
Which Vehicles Need Additional Cooling?
Which vehicles need fuel cooling and which do not is largely dependent on the expected drive cycle. The fuel temperature rise is strongly dependent on engine speed and only slightly dependent on torque. Higher speed engines have a greater need for fuel cooling as the fueling parameters must be controlled in a tighter time span and crank angle. The same quantity of fuel must be injected in a much smaller time increasing the work input to the fuel.
It is important to consider the drive cycle impact on the need for fuel cooling. If the intended drive cycle only requires cooling for transient applications, then there is the potential for the thermal capacitance of the system to partially cool the fuel. This is especially true as the transient temperature limit at the injection pump is often somewhat higher than the steady state value. Of course, the steady state problem cannot rely on any thermal capacitance to reduce the fuel cooling need. Those engines that spend much of their time operating in low gear, high RPM often have a need for steady state fuel cooling. This include combines and delivery trucks. Passenger cars do well have such a need, especially when they are used for freeway driving conditions.
In sizing the fuel cooling system, it is important to take into account the thermal resistance of the rest of the fuel system. This includes the lines and especially the tank. Depending on local ambient conditions, these components can act as fuel coolers or heaters, and this must be quantified. Decisions can be made that will increase the cooling benefit from these components. For example, if the tank is acting as a fuel cooler, then it is beneficial to design the in-tank canister to mix the return fuel with the bulk fuel as much as possible. On the other hand, if the tank is acting as a heater (air temperature near tank is hotter than bulk fuel), then a non-mixing canister is desired.
STRATEGIES FOR FUEL COOLING
There are several strategies for fuel cooling that can be pursued for automotive applications. Each has their own advantages and disadvantages as outlined below. If the need for fuel cooling is identified early in the design of the engine, then it is possible to engineer solutions that deliver many benefits that otherwise be available. For example, if a fuel recirculation loop is employed to the clean side of the filter, the fuel filter can be made much smaller due to the reduced flow. This also will greatly minimize the temperature rise in the tank.
Air Cooled Fuel Coolers
Cooling diesel fuel with air often has the appeal of being considered the lowest cost, least impact solution. The limiting fuel temperature is that of the ambient air, which is often the lowest temperature heat sink on the vehicle that is easily available.
The primary disadvantage is that the cooler must be mounted in a 'crash safe zone,' precluding installation in the radiator package. This limits the permissible packaging space to behind the front face of the engine or under the body of the vehicle. In both of these areas, there are generally high ambient temperatures and low air velocities. These factors cause the heat exchanger to be large relative to the space available. If the fuel cooler is to be mounted in the engine compartment, an electric fan or air ducting to the front of the engine or both may be required, adding greatly to the system cost and packaging space. These additional components supply the air at an appropriate velocity and temperature.
Similar problems occur for fuel coolers mounted underbody. Air flow here is strongly dependent on many factors, including body shape and design, wind direction, and vehicle speed. It is possible, however, to control the air flow to the cooler through suitable ducting of the ambient air. This should be done in such a manner that prevents it from mixing with the radiator outlet air stream. Ideally, the cooler should stay forward and low on the vehicle underbody.
The primary disadvantage of attempting to use engine coolant as the heat sink is that the fuel cannot be cooled below the outlet radiator temperature, which is around 100 °C. This is higher than the optimum 45 °C. fuel inlet specification on current rotary distributor and common-rail fuel injection pumps.
Fuel cooling is a definite need for advanced diesel engines based on fuel pump design and anticipated drive cycle. Several paths are available to the engineer in choosing an appropriate strategy to cool the fuel.
The choice to be used will be dictated by the fuel temperature required, the stage in the design cycle, package space and of course, cost. The engine coolant system offers many benefits including small size, ease of integration, low cost and eliminating the need for the fuel heater. However, the achievable fuel temperature is above the current limit specified by the fuel pump manufacturer. Air cooled solutions are technically possible, but are bulky and difficult to package.
The fuel inlet temperature has a significant influence on the performance levels of internal combustion engines, and this is particularly true for diesel engines. As a result, many passenger car manufacturers take this into account and incorporate a fuel temperature control system into their high-performance, latest-generation diesels.
HSDI passenger cars are utilized around the world, often operating in extremely high and low ambient temperatures. As a consequence, the engine's power and torque levels can be reduced to such an extent that the machine's performance is severely limited.
Decisive for these requirements are the results of optimizing air intake, engine breathing, fuel injection and combustion chamber geometry, all of which affect the combustion process. Diesel
engine development concentrates on such aspects as varying the shape of the combustion
chamber and the size of the piston bowl in conjunction with precise control of the fuel injection timing, pressure and quantity. Particular progress has been achieved with electronically controlled engines, and, in the process of minimising gaseous emissions, it has been possible to maximize performance levels.
The conflict of interest which engine developers are often faced with is how to improve one particular criterion without negatively affecting others. An efficient combustion process requires a homogeneous mixing of the charge air and the injected fuel. This process is assisted if the diesel fuel is heated. A higher fuel temperature improves the diesel fuel's fluidity (kinetic viscosity), which is important for the injection process and the mixing characteristics. An improved kinetic viscosity lends itself to an optimum formation of the injection spray, which in turn leads to reduced gaseous emissions, significantly influenced by the evaporation properties of the injected diesel spray.
With a starting point of 100 % power with the reference fuel temperature of 45 °C, without cooling, without heating, HSDI engine performance values decrease or increase, as the fuel temperature rises or falls respectively.
Of particular interest is the initial fuel heating of 10 °C from 45 °C to 55 °C, Which leads to a significant performance reduction of 7.5 %. A drop in the diesel fuel inlet temperature from 45 °C to 25 °C results in a performance increase of 5 %, and raising the fuel temperature from 45 °C to 80 °C leads to a performance reduction of 14 %. Performance continuously falls by 19 % with rising fuel temperature (from 25 °C to 80 °C), but the fuel consumption remains constant throughout.
The injected fuel volume falls with rising temperature, which is the direct result of a decreasing diesel fuel density with rising temperature. The lowest fuel consumption occurrs at a fuel temperature of 45 °C. As mentioned above, this is the reference temperature used in the diesel engine industry for establishing combustion processes.
The injected energy quantity continually falls with rising fuel temperature, from 25 °C to 80 °C. The specific fuel consumption also stays relatively constant, the best point being at 25 °C fuel temperature and increasing insignificantly up to 45 °C. Compared with the absolute fuel consumption, the lowest calculated specific fuel consumption occurs at 25 °C, as a result of the power increase of 5 %.
HSDI engine fuel temperature tests demonstrate clearly that changing the fuel inlet temperature from the standard temperature of 45 °C has a significant effect on performance levels. From 45 °C to 80 °C, the power drops by 14 %, and when the fuel is cooled from 45 °C down to 25 °C, performance increases by up to 5 %.
With this fundamental characteristic (increasing fuel temperature results in decreasing
engine performance) fuel consumption does not decrease at the same rate, but remains virtually constant. The reduction in engine performance with increasing fuel temperature is predominantly due to the accompanying decrease in fuel density, which in turn results in reduced injected fuel quantities and therefore energy content. The combustion process in the engine clearly is less efficient at higher fuel inlet fuel temperatures.
State-of-the-art HSDI passenger car engines are developed with a standard reference fuel inlet temperature of 45 °C.
The Giulietta diesels don't have a fuel cooler, but I'm considering installing one as it's easy to go above 45°C fuel temperature at the inlet of the common rail pump, especially in summer. The heat sink capacity of the fuel in the tank depends on the level, obviously. And there's no way of knowing how fuel lubricity is affected above 60°C as the standardised test by HFRR isn't useable above that temperature (the lightest fractions of the fuel evaporate leaving only the best lubricating highviscous parts, skewing results).
It could be worthwile to invest in fuel coolers if you're after that last few BHP without compromising durability, especially if you're at the limit your injection pump can deliver.
Anyone willing to monitor fuel temps for a while, I lack the ability and probably will for another 10 months...