Dynamic Cylinder Deactivation with Residual Heat Recovery

Abstract

Cylinder deactivation is a proven solution to improve vehicle fuel economy. This proposal presents Dynamic Cylinder Deactivation (DCD) solution to the conventional automotive engines. DCD is an engine cylinder deactivation solution based on engine thermodynamics and residual heat recovery. It is an innovative solution toward engine fuel conversion efficiency, totally different from traditional sealed-valves cylinder deactivation solutions. DCD has many advantages over traditional sealed-valves cylinder deactivation. Thermodynamic efficiency gain and residual heat recovery are the most attractive features from DCD advantages.

In this DCD proposal, method and theory of DCD with residual heat recovery are presented. Thermodynamic analysis is made to demonstrate the benefits of DCD and its related residual heat recovery. Electronic implementation is completed by microcontroller based control module. A top level electronic design is proposed, with multiple automotive grade devices from Infineon being applied into the control module. A thorough comparison between the proposed DCD and traditional sealed-valves cylinder deactivation is also presented.

INTRODUCTION

This proposal presents Dynamic Cylinder Deactivation (DCD) solution to the conventional automotive engines. DCD is an engine cylinder deactivation solution based on engine thermodynamics and residual heat recovery. It is an innovative solution toward engine fuel conversion efficiency, totally different from traditional sealed-valves cylinder deactivation solutions.

Traditional sealed-valves cylinder deactivation solutions began in 1980s ^[1]. It deactivates partial engine cylinders in a fixed pattern to reduce pumping loss, thus helps to increase engine fuel conversion efficiency. The big problem is it causes heavy engine thermal unbalance, with the deactivated cylinders being cooler than normal and the active cylinders being hotter than normal due to heavier unit cylinder load. As a result, the cooler deactivated cylinders will suffer from reduced lubrication and thermodynamic loss, as well as larger friction, mechanical worn-out and gas blow-out; while the hotter active cylinders will trend to knock and generate NOx emissions.

DCD deactivates all the cylinders inside the engine dynamically and in a balanced way. That is, all the cylinders inside the engine will be working in an intermittent mode, being activated and deactivated alternatively. The result would be not only a well balanced engine thermal condition under which engine performance could become the best, but also the residual heat recovery by engine thermodynamic expansion during the deactivation cycles. Based on all of these benefits, we could expect DCD would bring us higher engine fuel conversion efficiency than traditional sealed-valves cylinder deactivation.

BENEFITS TO COMSUMERS

Energy conservation is the best way to solve energy problem. Increase engine fuel conversion efficiency is an effective way to implement energy conservation. Most motor vehicles require fossil fuel as energy source. In US, motor vehicles consume 69% of fossil fuel energy. It is believed that much of benefit would come from fuel efficiency improvement. A 10% efficiency improvement in vehicle performance would save over $82 billion US dollars per year to import foreign oil based on the current $120 crude oil price, and reduce emissions of carbon dioxide by 171 million metric tons per year.

Cylinder deactivation is a proven solution to save fuel. It has been adapted by majority of automobile manufacturers. General Motors’ cylinder deactivation solution is called Active Fuel Management (AFM). It gives a 6% to 8% improvement in fuel economy ^[2]. DaimlerChrysler’s cylinder deactivation solution is called Multi-Displacement System (MDS). It claims that fuel economy will boosted by 10% to 20% ^[3]. Mercedes-Benz’s solution is called Active Cylinder Control (ACC) ^[4], it was applied to its V12 engine only. Mitsubishi also had MD System (Modulated Displacement) ^[1] in 1982 based on its 4-cylinder engine. Honda’s solution is Variable Cylinder Management (VCM) ^[5]. Facing the current $120 crude oil price, more and more vehicles have been and will be integrated with cylinder deactivation solution.

All of the above solutions are referred as traditional cylinder deactivation solutions. They all utilize the method of disabling and sealing the valves of the cylinder to be deactivated. Normally they are implemented mechanically by hydraulic or electromagnetic valve actuation controls.

To automobile consumers and vehicle drivers, the benefits of cylinder deactivation is simply reduced fuel consumption and improved fuel economy. It also helps to reduce engine emissions and CO2 discharge. Saving fuel means energy conservation, which will help to solve energy problem and ease the crude oil price. It also has positive contribution to the public society by reducing global warming and greenhouse effects.

As will be seen below, DCD would have even more benefits than traditional sealed-valves cylinder deactivation.

THEORY OF OPERATION

DCD is an electronic based cylinder deactivation solution. Controlled by electronic circuits and microcontroller, it deactivates all the cylinders inside the engine dynamically and in a balanced way. That is, all the cylinders inside the engine will be working in an intermittent mode, being activated and deactivated alternatively. The result would be a well balanced engine thermal condition under which engine performance could become the best, and also residual heat recovery by engine thermodynamic expansion during deactivation cycles.

DCD will not disable and seal the valves like what is being done in all traditional sealed-valves cylinder deactivation solutions. Instead, its deactivation will be applied cylinder by cylinder and cycle by cycle in an intermittent way, with the consideration of both mechanical balance and thermal balance. It enables or disables the cylinder by turning the fuel injection on or off. As a result, engine’s deactivation duty cycle will be tightly controlled by electronic DCD controller’s duty cycle, which could be switched in fine pitches and in predetermined durations.

The electronic DCD controller of the proposed DCD is very straightforward. It simply cuts fuel injection off to deactivate certain cylinder(s) at a single engine cycle, and keeps fuel injection on to activate the other cylinders during the same engine cycle, as well as turns fuel injection on to reactivate the deactivated cylinder(s) during the next engine cycle(s). For this purpose, deactivation patterns could be generated for desired deactivation duty cycles with balanced operations. An example deactivation pattern is shown in Table 1 below. It is designed for a 4- cylinder engine with a repeat cycle of 3, resulting deactivation duty cycle of 33 %. This means each of the 4 cylinders will be turned off once during 3 engine cycles. For 4-stroke engines, each engine cycle covers 2 crankshaft revolutions. So this deactivation pattern would be repeated in every 6 engine revolutions, and every cylinder will be deactivated once evenly in every 6 engine revolutions.

In the above pattern, the fuel injection events to be deactivated are highlighted with green, while other non-highlighted events belong to normally activated cylinders. Engine crankshaft angle (CA) is increasing in vertical direction. From upper to lower, crankshaft makes 2 revolutions or 720 degrees for one full engine cycle. The numbers in vertical sequence just reflect cylinder numbers and their ignition order. From left to right, engine cycles are listed horizontally. We need to deactivate 4 cylinders in 3 engine cycles, which contains 12 fuel injection events. Since the deactivation duty cycle is 33 %, 4 among 12 fuel injection events must be turned off. This means to turn off one event after every 2 active events. So the space between 2 deactivations is 2 events. At the first cycle, if cylinder #1 is deactivated, then cylinders #3 and #4 will keep activated; and then, cylinder #2 will be deactivated. Next, the engine goes to the second cycle, cylinder #4 will be deactivated only after cylinders #1 and #3 have been activated. Still next engine cycle, cylinder #3 will be deactivated only after cylinders #2 and #1 have been activated, and so on….. This pattern will be repeated as long as the deactivation is in action.

The balance of the deactivation pattern is very important. The balance in engine timing sequence will result smooth mechanical operation. The balance in deactivation duty cycle will cause balanced cylinder thermal condition and balanced cylinder temperature. All these balances will keep engine operating in a perfect condition, thus providing higher fuel conversion efficiency.

Once the above deactivation pattern is implemented, another great benefit to come is residual heat recovery. In the above example pattern, before each deactivation, every cylinder has been activated normally for 2 engine cycles, its temperature has reached normal operation level. During the deactivation cycle, cold air would be sucked into the hot cylinder as usual, being compressed and heated up by the residual heat left by the previous active cycle(s), and then expanded with the residual heat energy absorbed from the cylinder. In other word, such a standard 4-stroke engine cycle still has a source of heat addition from the remaining heat energy contained in the cylinder. As a result, the deactivated cylinder would still contribute some positive mechanical work based on the residual heat energy contained in the cylinder. Such kind of new engine cycle is originated from concept of High Efficiency Integrated Heat Engine (HEIHE) ^[6], and can be referred as HEIHE cycle.

THERMODYNAMIC ANALYSIS

Traditional sealed-valves cylinder deactivation compresses and expands gas repeatedly in sealed cylinders. The only benefit of such sealed cylinders is reducing the gas pumping loss in two folds, with the first fold coming from reduced engine power that requires wider throttle opening, resulting higher intake manifold pressure; the secondary fold coming from the sealed cylinders that demand no gas flow, resulting even higher intake manifold pressure. However, compression process in sealed cylinder during deactivation will generate heat and rise the temperature. Once the gas temperature goes higher than that of cylinder wall or the engine coolant, the heat will spread out, or be carried away by the coolant. So during the compression the existing energy inside the cylinders will escape in the form of heat, causing thermodynamic loss. As a result, the expansion after the compression will be less energized, yielding less expansion work than compression work. The overall work done during a compression-expansion process could be a negative one, and such negative work would happen twice during the whole 4-stroke engine cycle. If we consider sealed cylinders as air springs, then these air springs would not bounce back as powerful as they were compressed due to the heat loss.

Even though the hot exhaust is sealed in the cylinders at the beginning of deactivation, as many automakers are doing so, the thermodynamic loss will extract their heat energy out of cylinders stroke by stoke and cycle by cycle, reaching a cooler than normal temperature eventually. Cylinder with cooler than normal temperature will suffer from many unpleasant issues like reduced lubrication, increased friction, mechanical worn-out and gas blow out.

In contrast, DCD would keep the gas flow through the cylinders as usual. So its gas pumping loss reduction benefit only comes from wider throttle opening, the first fold mentioned above. Obviously, there will be no benefit from the secondary fold mentioned above because of the gas flow. However, the great benefit of DCD comes from thermodynamic expansion of the gas inside the cylinders.

Before the scheduled deactivation cycle, the cylinder to be deactivated have operated actively as usual at least one engine cycle, with heat addition by fuel injection(s) and combustion(s) as usual. Thus the temperature of the cylinder would be brought up to the normal, or close to the normal. During the scheduled deactivation cycle, fuel injection of the deactivated cylinder would be cut off electronically, but the cylinder operation cycle would remain in original 4 strokes as usual.

During the intake stroke, cold fresh air from atmosphere with environment temperature is inhaled into the cylinder. Then it is compressed during the compression stroke. The gas temperature would be raised not only by the compression, but also by the remaining heat from the previous combustion(s). Next would be the expansion stroke, the heated compressed gas would expand inside the deactivated cylinder, pushing the piston downward while contributing a positive mechanical work. Due to the residual heat energy inside the cylinder, more expansion work is expected than the work spent for compression. This means the heat energy would be converted into mechanical energy through gas expansion. At last, the expanded gas would be discharged out of the cylinder during the exhaust stroke with much lower temperature. Some heat rejection would happen during exhaust process, this is a must process for the operation of heat engine which always has less then unity efficiency.

After the scheduled deactivation cycle, the cylinder that has been deactivated would be reactivated as usual at least one engine cycle, with heat addition by fuel injection(s) and combustion(s) as usual. Thus the temperature of the cylinder would be brought up to the normal, or close to the normal. The more reactivated working cycles, the closer the temperature of the cylinder would be brought up to the normal, ready for the next deactivation cycle.

Thanks to the HEIHE cycle happened above, DCD would definitely have a positive thermodynamic gain as long as the cylinder is hot enough. Based on the fact that every cylinder does have some residual heat after normal combustion(s), the positive thermodynamic gain from DCD could be irrefutable. This gain would greatly contribute to engine fuel efficiency.

During the process of 4-stroke engine cycle, the gas flow will be kept inside the cylinder for half of the stroke period, on average, during the intake stroke; and full stroke period during the compression stroke and the expansion stroke. This results up to 63 % on average, 75 % maximum, engine cycle time for the gas flow to stay inside the cylinder, getting in touch with cylinder wall and being heated up by the cylinder before and during the expansion work.

ELECTRONIC IMPLEMENTATION

In order to implement DCD, an electronic DCD controller module must be inserted between Engine Control Unit (ECU) and fuel injectors, so as to disable some of fuel injection pulses to fuel injectors according to deactivation pattern. Fig. 1 shows the system block diagram of DCD control module. It is an add-on control module to most existing vehicles. Note that original electrical connections between ECU and fuel injectors must be cut and separated as indicated by red “X”, and then insert the DCD control module in between. To close fuel control loop under new oxygen balance caused by DCD, a wide band O2 sensor must be used to detect engine exhaust gas flow. Wide band O2 sensor signal would be processed by DCD control module, and be converted into the format acceptable by vehicle ECU. LSU-4.2 type wide band O2 sensor manufactured by Bosch with part number 0-258-007-057 ^[7] is recommended for this application. It could sense as wide as 21% of wide band oxygen content, the O2 content of pure air.

Fig.2 shows the block diagram of DCD control module. Basically it is implemented by an 8-bit microcontroller. Infineon’s SAK-XC886-6FFA5V 8-bit single-chip microcontroller ^[8] is selected for this application. It has up to 34 digital I/O ports and up to 8 analog input ports, more then enough for DCD application. It has embedded with 24k-byte flash memory and 1792-byte SRAM. This microcontroller is manufactured in a standard PG-TQFP-48 package, with -40°C to +125°C automotive grade temperature range which is most suitable for various kinds of automotive applications. XC886 family microcontroller has 2 different working voltages, 3.3V and 5V. Its 5V version is used for the present proposal. It is also powered by a self-contained 2.5V core voltage, eliminating the requirement of extra power regulator.

DCD control module is designed to support up to 8 cylinders with 8 fuel injectors. Fuel injector control signals from ECU are isolated by optical couplers before entering XC886, with correct logic input level at 5V. Vehicle speed signal is also fed into the XC886 through optical coupler in the form of digital pulse. Both engine temperature and wide band O2 sensor signals are fed into XC886’s analog input ports in analog format. Next, XC886 makes A to D conversion and then processes them in digital domain. XC886 also accepts control signals from keypad and setup or configuration signals from DIP switch.

XC886 will control all the fuel injectors with the required DCD control patterns. Fuel injector control signals from XC886 are buffered by a bank of low side drivers. Infineon’s TLE6220GP smart quad low side switch ^[9] is selected for this application. It provides up to 60V 3A driving capability, with 320 m turning-on resistance. TLE6220GP also possesses short circuit protection, over temperature protection and over voltage protection features which are desired for automotive applications.

XC886 also drives a 2-digit numerical display for presenting control statues and information to the user, along with other 4 LEDs. At last, XC886 provides a PWM output to ECU’s O2 sensor input through an RC filter, so that ECU could still close the fuel control loop as usual under DCD control.

A DC/DC converter is used to convert +12V to +14V automotive power into +5V power rail required by XC886 microcontroller and all other devices. Infineon’s TLE6389-2GV50 step down DC/DC controller ^[10] can be used for this application. It supports up to 2.3A output current, with fixed +5V voltage. TLE6389-2GV50 accepts all kinds of automotive power inputs from +5V to +60V, it also works under automotive temperature grade.

DCD control module will be powered on whenever engine ignition switch is turned on. However, DCD control function could be enabled and disabled manually by the user.

Fig.3 shows the front panel layout of DCD control module. Numerical display is placed at the leftmost. By default the numerical display shows the current DCD control duty cycle in percent. This data also reflect the percentage of engine power decrease under DCD control. Right close to the numerical display are “UP” and “DN” keys for adjusting the target DCD control duty cycle. A large toggle key is placed at rightmost. This key is used for manually enable or disable DCD control function. The default state of this key is DCD control function disabled, with its LED indication off and numerical display shows “00”. 3 function keys are placed in the middle, once one of them is pressed, the related LED indication will be on for 3 seconds, along with the related data is shown on the numerical display for 3 seconds. “STEP” means time in second to change DCD control duty cycle for one step whenever the DCD control duty cycle is being increased step by step from default zero percent to the preset target percentage. During the “STEP” display time, multiple presses to the “STEP” key could change the duration of the control step. “SPEED” and “TEMP” functions are just used to monitor the vehicle input signals or for reference, they are not direct indications for DCD control.

The conditions required to start DCD control are as follows:

DCD control enable key is turned “ON”, with its LED indication on;
Engine has been warmed up, with coolant temperature higher than certain level;
Vehicle speed is within DCD control range, 40 MPH to 70 MPH for example.

Once DCD control is started, DCD control duty cycle will be increased from zero or a very small value to the preset target step by step, each step will spend a preset time duration, 3 seconds by default, so as to change DCD control duty cycle gradually and smoothly. Numerical display will show the real time value of the DCD control duty cycle.

Once DCD control duty cycle has reached the preset target, it will be hold onto the target without change, unless “UP” or “DN” keys are pressed and target value is changed.

Deactivation patterns for different duty cycles and different engines will be converted to data blocks and stored in the flash memory of X886 microcontroller. During DCD control, the related data block is checked out according to the current DCD control duty cycle. The data block will tell microcontroller whether to turn on fuel injection or to turn it off. For turning on fuel injection, just simply copy the fuel injection inputs to their outputs, without altering their original timing and duration determined by vehicle ECU. For turning off fuel injection, just simply block the current fuel injection pulse, sending no signal to the output.

A DIP switch is used to determine the configuration setups:

Number of cylinders, 4, 6 or 8;
Maximum allowed DCD duty cycle, 50 % or 67 % for example;
Coolant temperature to start DCD control;
Vehicle speed range for DCD control;
Unit for speed display, kmPH or MPH;
Scaling system for temperature display, Fahrenheit (in first two of 3-digit) or Centigrade.

SUMMARY

Cylinder deactivation is a proven solution to improve vehicle fuel economy. Dynamic Cylinder Deactivation (DCD) has many advantages over traditional sealed-valves cylinder deactivation. Thermodynamic efficiency gain and residual heat recovery are the most attractive features from DCD advantages. Its overall performance over traditional sealed-valves cylinder deactivation, both mechanically and thermodynamically, can be summarized with the following Table 2.

Based on the above comparisons, we could expect that a new kind of cylinder deactivation, in terms of theory, methodology and modular apparatus, is coming to human life. That is Dynamic Cylinder Deactivation (DCD) with residual heat recovery proposed in this document. The innovative HEIHE cycle involved within the present DCD proposal will bring extra engine efficiency gain over traditional sealed-valves cylinder deactivation. Due to its simple electronic implementation, DCD control could not only be applied to the manufactured vehicles, but also be made into after market devices or add-on control modules for retrofitting millions of existing road vehicles.

REFEREENCES

Wikipedia.org, Variable displacement.
Wikipedia.org, Active Fuel Management.
Wikipedia.org, Multi-Displacement System.
Wikipedia.org, Active Cylinder Control.
Wikipedia.org, Variable Cylinder Management.
Heihetech.com, High Efficiency Integrated Heat Engine (HEIHE), 2007.
Bosch, Planar Wide Band Lambda Sensor Data Sheet, in Bosch_LSU4.2_data_english.pdf.
Infineon, XC886/888CLM 8-Bit Single-Chip Microcontroller Data Sheet, V1.0, May 2007.
Infineon, TLE6220GP Smart Quad Low Side Switch Date Sheet.
Infineon, TLE6389 Step Down DC/DC Controller Data Sheet.