Meddle With the Pedal: ELECTRONIC THROTTLE CONTROL
Doing away with the throttle cable was just the beginning. Electronic throttle control (ETC) has allowed engineers to add many other noteworthy vehicle systems and capabilities, with more to follow.
It may well be that software integration of automotive electronic systems will turn out to be the most significant automotive technological development of this decade. Originally electronic systems such as ABS, HVAC and emissions were developed separately by those groups within each carmaker that were most responsible. The brakes and suspension group worked on ABS while emissions and engine control issues were handled by powertrain people. Software integration has brought these systems together. The result is new, interrelated technologies that produce better mileage, safer cars and reduced emissons.
At the head of this trend, as an enabling technology, is electronic throttle control (ETC), which is part of an industrywide response to calls for better fuel economy, reduced emissions and a reduction in vehicular fatalities. This story is not so much about hardware as it is about software that uses ETC as an input and an actuator to make the new technologies possible.
Without ETC, the planned advances in hybrid and diesel technology that are now right around the corner would not be possible. Current advances such as electronic stability control (ESC), expected to save thousands of lives per year, would simply not be possible without ETC. Best yet, ETC reduces cost and complexity for carmakers by integrating formerly stand-alone features such as idle control, cruise control and throttle control into a single, mostly software-based system.
This latest version of electronic throttle control should not be confused with the earlier stand-alone systems that replaced the mechanical link between the driver and the engine. In these new systems, the output of the pedal sensor is an input not only to the engine control system but to the software system as a whole. As such, pedal angle becomes a valuable input to other electronic control systems. The algorithms’ that control the ABS, ESC, cruise control, HVAC and other system functions all use pedal angle data in the decision-making process. The throttle angle that results is not only what the driver wants but what the systems needs for correct and safe operation.
In these new-generation ETC systems, the accelerator pedal module becomes a two-way device: It, accepts information about desired engine output from the driver, plus it can feed back tactile information to the driver as a warning that the selected engine output is either wrong or dangerous.
The Technological Need for ETC
The clear goals of the automotive industry are to improve fuel economy, reduce emissions and improve function and safety for the driver. To understand the design options available to accomplish these goals, you need to know what produces the best results and what causes subpar performance. These complicated goals are further complicated by trade-offs that have to be made.
Fuel economy and emissions output per mile traveled are directly related to the size of the vehicle and the size of the engine. In keeping with the laws of physics, cutting fuel consumption is about either reducing the mass of the vehicle or reducing acceleration. Since the systems are not perfect, there’s another path that can be traveled-by improving efficiency to reduce losses.
The first thing to know is that most automobile engines are much larger than they need to be for most real-world operating conditions. The big V8 often selected for full-size pickups is really chosen to pull a boat or trailer the owner may have in mind. Yet trailer towing may amount to less than 10% of the actual vehicle miles; 90% of the time a smaller engine would do just fine.
The fact that engines generally spend most of the time running at a small fraction of their peak power output is referred to as the partial power problem. Toyota says the Otto cycle engine is most efficient at 40% to 45% of its redline rpm. This is the point at which torque is at about 70% to 80% of its peak value for a given engine. In this most efficient operating range, the engine produces about 40% of its peak power rating.
Let’s use Toyota’s 108-hp ECHO engine as an example. Given the numbers just mentioned, it would be best if most of the time the engine output were in the range of 40 to 50 hp. Unfortunately, this is not enough for adequate acceleration or hill climbing. Calculations show that if the ECHO had only a 30-hp engine, it would need 30 seconds to accelerate to 60 mph. If such a vehicle were to encounter a 10% grade, it would slow down to 30 mph before it reached the top of the hill.
On the other hand, only 15 hp or so is needed to maintain 60 mph on level roads, and even less power is needed for idling and low-speed travel. The net result is that the engine power output that was chosen for adequate passing and hill-climbing is larger than necessary for most of the operating circumstances of the vehicle.
In addition, engines are seldom operated under the circumstances that would produce the best results for fuel economy and emissions output. For a typical engine with a redline of 5000 rpm, the sweet spot should be at about 2000 rpm. In practical terms, most engines actually operate in a much broader range between idle and 3200 rpm. There are the occasional zooms to redline, but they represent a small part of the true operational circumstances.
Given normal gearing, the peak efficiency point for a vehicle turns out to be around 55 mph. The double-nickel speed limit was not chosen randomly, but rather with an eye to best fuel economy for the average vehicle. For a given distance traveled, fuel economy tapers off at both higher and lower speeds. Holding a constant speed is an advantage, as it avoids both the extra fuel needed for acceleration and the increased emissions that often result from deceleration.
Not suqmsingly, the sweet spot of engine efficiency is also the sweet spot of emissions output. It’s the cold-start events and sudden speed changes that challenge emissions control systems. Electronic throttle control can actually help emissions through strategies that lean out the mixture in concert with retarded ignition timing to assure an earlier light-off for the converter.
Efficiency losses occur on both sides of the sweet spot for a given engine. At high engine speeds, friction among the piston, the rings and the cylinders accounts for more of an engine’s lost output. These friction losses become more significant as engine size is reduced. Parasitic losses to engine accessories such as the oil and water pumps also increase as a function of rpm. Another issue is the need to riehen the fuel mixture to get maximum torque output from the engine. It may help acceleration but it doesn’t help emissions output or fuel consumption.
The major cause of efficiency losses at low speed is called pumping kiss. Reducing the output of an engine is accomplished by limiting the airflow into the engine. The throttle plate restricts the intake of air by forcing the engine to drag air through a narrow or restricted inlet. The restriction of the air intake creates a differential pressure across the throttle plate we know as intake manifold vacuum. Since the air entering the cylinder is below atmospheric pressure, less air enters the cylinder. The engine control system measures the pressure differential and reduces fuel input accordingly. The reduced quantities of air and fuel result in the desired reduction of power output.
The downside to this is that having partial pressure in the intake manifold wastes energy. As the piston moves downward on the intake stroke, normal pressure below it and partial vacuum above it cause drag on the rotation of the crankshaft. These pumping losses occur during most engine operating conditions, as the throttle is seldom truly wide open.
Diesel engines are known to be approximately 25% more efficient than gasoline engines. According to Toyota, one reason is that die diesel engine uses no throttle, and thus suffers reduced pumping losses. In gasoline engines, the throttle-related losses are !relieved to be in the range of 7% to 10%. Diesel engines are also more efficient due to their higher compression ratio.
GM says that it’s difficult to achieve all of the design goals of better fuel economy, reduced emissions and driver safety at the same time. Typically, in a fixed-valve-timing engine, best power has be traded off against other desirable elements such as torque, idle stability and fuel economy.
There are other approaches that try to deal with the issue of the throttlerelated partial power problem. Gasoline direct injection is an approach to improving efficiency by calibrating each combustion event to the needed power requirements. The direct injection system controls engine power by injecting only that amount of fuel needed to produce the desired engine power output.
Another approach is through variable valve timing. WT systems offer varying degrees of control based on system complexity limits. Early intake valve closing (EIVC), late intake valve opening (LIVO), late intake valve closing (LIVC) and fully variable valve lift strategies have demonstrated reduced pumping losses and improved fuel economy.
CM has tried the EIVC strategy, which uses the variable intake valve closing and intake valve lift control to unthrottle the engine at part-load and light-load operating conditions. Here the intake valve duration and lift are significantly reduced to control airflow into the engine, allowing it to operate at higher intake manifold pressures with the potential to fully unthrottle the engine under all operating conditions.
Electronic or hydraulic valve actuation solenoids under the direction of software-controlled cam profiles may someday offer even greater flexibility. These systems have been talked about and demonstrated. Renault had (me in a Formula One racer a wliile back with a 17,(XX)-rpm redline. So far the software cam has not appeared in a production vehicle due to the dynamic complexity of landing the valve back on its seat without noise. The actuators shown so far are also bulky and expensive in comparison to mechanical actuation.
According to GM, the downside to these various WT strategies for production engines is that they require moderate to significant changes to the engines architecture to successfully package the WT components. Cam phasers not only take up space, but also add to the vehicle in ternis of complexity, weight and cost.
Using ETC to Achieve System Goals
Automakers have taken this research into engine efficiency and done their best to make sure the engine spends more of its time in and around the sweet spot. They know that the vehicle has to feel “normal” to the driver and have gone to great lengths to make that happen. From the driver’s standpoint, what the computing platforms are adjusting and controlling is strictly in the background. Getting greater efficiency is accomplished in several ways:
Transmission Control. Keeping die engine at its rpm sweet spot is accomplished by having more of the need for rpm compensation between the engine and the drive wheels handled by the transmission. Six-, seven- and eightspeed transmissions, as well as CVTs, are becoming commonplace. As CVTs are still limited in their peak torque handling capability, vehicles with highoutput engines have stayed with conventional multispeed transmissions.
Ford and GM are in production now on their joint venture six-speed transmission. About 85% of the components are meant to be shared by 10th manufacturers. Expectations are for a 4% increase in fuel economy while at the same time providing a 7% improvement in O-to-60 times. Unlike conventional transmissions, with their ratio spread of approximately 4.0 to 1.0, the new Hydramatic/Ford transmission has a wider overall ratio of 6.0 to 1.0. Electronic throttle control is an integral part of the improvements in both fuel economy and acceleration.
Having so many gears requires some adaptations. Toyota’s eight-speed transmission, for example, has a software provision to skip gears during deceleration to make the downshifting smoother and less apparent to the driver. The ETC system smoothes the shift performance between gears by adjusting the throttle opening at the shift point. Programmed steps in the ETC system can be used to give the driver the “feel” of a conventional transmission, so the CVT doesn’t feel odd.
Driveline management software is used to select the combination of engine output and gear ratios that WUl deliver the needed torque in the most efficient way. The software is capable of reducing the torque input to the transmission during the shift sequence to reduce mechanical shock to the drivetrain. This driveline management software is especially important for on-demand AWD. The shift from two-wheel to four-wheel drive must lie controlled to avoid torque bumps and other interactions between the (Irive wheels.
Displacement Reduction for Light-Load Conditions. GM, Chrysler and others have implemented variable displacement strategies. GMs Displacement on Demand system reduces effective engine size during steady-state, lowpower conditions. A key aspect of this system is the ETC system’s ability to create a greater throttle angle without the need for the driver to change pedal angle. This is a key element to assuring that the only indication to the driver that four of the V8’s cylinders are deactivated is the light on the dash.
Emissions Control. ETC is a part of the strategy to reduce engine emissions during cold start-up. One way to cause quick heating in the catalytic converter is to retard timing and lean out the mixhire. The ability to do this is limited by the loss of torque and power that results from these engine settings. For u given pedal position, the driver will feel reduced power when the strategy is implemented. Software compensations to the throttle angle can be made that maintain the original pedal-to-throttle relationship the driver is used to.
ETC also can be used to control actual throttle angle during acceleration and deceleration to minimize pumping losses. Often the throttle angle implemented by the ETC system could be more favorable than the driver Ls able to select.
The greatest impact on emissions performance of ETC systems is the above-mentioned variable displacement strategies. Cadillacs first attempt at variable displacement (V-8-6-4) in the early ’80s foundered at the time for a variety of reasons, including driver dissatisfaction with how the engine “felt” as it dropped or picked up displacement. ETC, with its computer control, is able to automatically make the throttle angle changes needed so the change is seamless to the driver.
Another benefit of ETC, according to CiM, is the ability to modify vehicle response to a change in pedal angle position. Consumer research shows that vehicle response to accelerator inputs greatly affects a driver’s overall satisfaction with the vehicle. The response of the vehicle to the first 2()mm (.8 in.) of throttle movement maybe more important than the actual 0-60 acceleration time.
Vehicle Safety. Electronic stability control is probably the most significant safety development since the invention of the seat belt. By federal requirement and antomaker cooperation, this system will be standard on all vehicles by 2010. The hope is that as many as 10,000 lives per year will be saved. To function as it does, ESC depends on ETC.
Electronic stability control systems are an integration of existing vehicle systems (ABS, TC, ECM), coupled with added sensors to determine steering angle and yaw. A key input to the system is the pedal angle position sensor output. The ESC system runs an algorithm that detennines if the requested engine output is safe. When it’s advisable, the output from the system can be a tiirottle angle command that’s not what the driver requested. When diere’s a possible loss of traction and/or steering control, the ESC system can overrule driver input to reduce throttle angle and engine power.
Electronic throttle control can also be used to protect the engine, driveline and tires from operation that may cause excess wear or damage. Rev limiting can be accomplished in software by governing the throttle angle rather than cutting off fuel or ignition. This results in a much smoother limiting that does not cause the driver to sense that the engine has “cut out,” as can be the case with ignition- and fuel-based limiter systems. Rental car companies have pushed for rev limiters as a way of protecting their assets from drivers who don’t care how hard they push a vehicle simply because they don’t own it.
Using ETC to Enable Other Technologies
GM’s Vortec 5.3L V8 uses Active Fuel Management (AFM), with ETC as a key input. The 3.9L V6 also uses AFM, but in combination with WT. GM says the 3.9 is the first to use both cylinder deactivation and WT on the same engine. Under light-load conditions, either engine can deactivate half the cylinders. Real-world fuel savings of 7% is what GM is advertising, although the benefit reportedly is greater for those who do a lot of steady-state highway cruising.
The E38 ECM measures load conditions based on inputs from vehicle sensors such as ETC and interprets that information to manage more than a hundred engine operations. Fuel injection, spark control and electronic throttle control are all included. When loads are light, the engine computer automatically closes both the intake and exhaust valves, while at the same time cutting fuel delivery. When the driver demands acceleration or increased torque to move a load, the cylinders are reactivated.
In these systems, ETC is used to balance torque to prevent the driver from “feeling” the cylinders as they come or go off-stream. During deactivation, both valves are closed. The energy used to compress the air in the cylinder is returned to crankshaft on the downstroke as the trapped air acts as a spring. The transition takes less than 20mS, and the driver never notices it.
The actual hardware used to control the deactivation is called a lifter oil manifold assembly (LOMA), and is located in the valley of the V8 engine. Four electric solenoids are controlled by the result of the E38’s processing of the load algorithm. These solenoids determine the number of active cylinders by controlling oil flow to the lifters of the affected cylinders.
In an AFM-equipped engine, pumping losses are reduced during deactivation primarily by the increase in intake manifold pressure. During deactivation, the remaining cylinders need reduced throttling in order to provide an equivalent amount of work. Without electronic throttle control, the driver would notice the deactivation as a sag in performance. Without the driver needing to change the pedal angle, the software changes the throttle angle to reflect the fewer number of functioning cylinders.
AFM operation is load-based. The load is measured and combined in an algorithm with the driver’s demand for power as measured by throtde application. Active fuel management does not affect emissions output from the active cylinders. For the inactive cylinders, no fuel is wasted or burned, and the result is lower emissions for the distance traveled.
The key point here is that the only mechanical components needed are the three or four special valve lifters and the solenoids to control them for the cylinders that are to be deactivated. The software-based control system uses inputs about engine load, vehicle speed, driver intention, safety and emissions inputs in making the decision to shut down individual cylinders. The ETC system already in place is used to make sure the vehicle operates “normally” during the deactivation.
The Gen IV Vortec 5.3L takes ETC to the next level by taking advantage of the processing capability available in the E38 computer. The increased integration allows the elimination of the throttle actuator control (TAC) module. In previous systems, the TAC module took commands from the ECM and operated the electric stepper motor that controls throttle position. In the new system, the ECM operates the dirotde direcdy. This direct link between the dirottle and the computer speeds up response time. Eliminating the TAC also reduces wiring, reliability issues and the need to monitor the TAC module for correct operation.
The flex-fuel 5.3 requires no special fuel sensor. Earlier flex-fuel engines used a light-reactive sensor to determine what blend of fuel was in the system. The Gen IV engine uses a virtual sensor programmed into its software. Based on readings from the oxygen sensors, fuel level sensor and vehicle speed sensors, the ECM determines the fuel blend and adjusts the fuel injector pulse width and the throttie angle as required. The ETC system makes the needed dirotde angle changes. Because ethanol has a lower BTU rating for the same volume as gasoline, more fuel is required to provide the same power at wide-open throttle.
Toyota is using what it calls Power Train Management on the Lexus LS 460. With this system, the most suitable vehicle drive power is accurately accomplished with optimized engine torque and gear ratio. Toyota’s emphasis is on what the driver experiences, which is torque at the drive wheels.
Toyota says that with conventional powertrain controls, a target throttle opening and gear ratio are determined according to the drivers pedal angle input. Consideration in terms of throttle angle is given to other vehicle systems such as cruise control and vehicle stability control (VSC). The result is that the target throttle opening and target gear ratio are set separately. In previous systems, this situation worked well because each of the vehicle systems was not large, and the desired accuracy requirements were not particularly high.
This situation has changed. Newer vehicle systems with precrash safety systems and intelligent parking assistance (IPA) systems have caused the relationships among vehicle systems to become more complicated. It has become more difficult to align all the different systems to achieve the desired drive power.
Toyota says it has developed something called Vehicle Dynamics Integrated Management (VDIM) to integrate the vehicle stability control, traction control, ABS and electric power steering. ETC is used for VDIM sensor inputs and control actuators. The VDIM system controls the “drive power” by selecting the combination of engine power and transmission gear to give the needed drive wheel torque at the highest possible efficiency. By having an integrated system, the best choices for ignition timing, engine rpm and gearing can be chosen to deliver torque and acceleration the driver senses.
The torque and power of the drivetrain for hybrid vehicles can come from the internal combustion engine, the generator and/or the electric motor. Combining and distributing the torque is handled by a planetary gearset that both Toyota and Ford call a power split device (PSD). In the PSD, the carrier gear is connected to the engine, the sun gear is connected to the generator and the ring gear is connected to the electric motor. The planetary gear configuration provides decoupling of engine speed from vehicle speed.
While a hybrid drivetrain offers the possibility of improved fuel economy, there are some added constraints. Ford says one issue is that power split vehicles are sensitive to such noise factors as engine torque mismatches that conventional vehicles are not. These systems are also sensitive to overuse of the battery that may affect its durability. To overcome these issues, Ford Escape/Mercury Mariner engineers had to determine powertrain operating points compatible with the battery and high-voltage bus architecture to ensure that power, voltage and durability issues were met.
Ford says that the determination of a desired powertrain operating point for a conventional vehicle is relatively straightforward, since there’s only one path to the wheels from the power-generating device (the engine). There are three variables that need to be determined-the transmission gear, the torque converter clutch state and the desired engine torque. The drivers intent is reported by the pedal angle sensor. The gear and torque are determined by computer algorithm, with die result that the throttle angle is controlled.
In a hybrid vehicle, diere are three power-producing devices-the generator, the motor and the engine. The control system determines what the driverdemanded wheel torque is by way of the pedal angle sensor. From this, the computer software can choose the optimum combination of desired engine speed and desired wheel torque. Engine speed is the result of the throtde position algorithms control of the throttle angle. Wheel torque is the result of the choice of power sources and the gearing between them and the wheels.
In the power-split hybrid electric vehicle, generator torque and generator speed-and, therefore, generator power-are largely determined by the desired engine speed and actual engine torque. So the battery power limit is essentially a constraint on motor power. Since motor speed is determined by vehicle speed, tnis effectively limits motor torque. Motor torque is also limited by what the driver wants in terms of driveability. The hybrid control system has to manage the interactions of the three possible power sources. Electronic throtde control integrated into the system is used to accept the input of the driver and to then control the engines output in accordance with the other two sources of power.
At the heart of the hybrid control system described by Ford is the electronic throttle control system and its ability to accept driver input and then output a throttle angle position in keeping with the best interests of the whole system. It’s the integrated software of the transmission and engine control systems that gives the system response.
To sum up, what started out as a means of eliminating the mechanical connection between the throttle pedal and the engine has evolved and taken on a larger and far more important role. By integrating the safety, emissions and powertrain electronic subsystems, it has become possible to implement new technologies that could not have been implemented independently. Electronic throttle control is a mandatory element of these advanced systems.
Copyright Hearst Business Publishing Jul 2007
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