This article was first published in 2001.
Electronic throttle control (aka ETC, E-gas and throttle-by-wire) has been fitted to high specification vehicles for over 10 years. BMW, for example, introduced a relatively simple form of electronic throttle control to its top-line 750iL in 1988. Following these early systems, there have been significant developments in safety strategies and an increasing complexity in the software and hardware.
ETC's variable relationship between the accelerator position and throttle blade position is ideally suited to compensating for an assortment of factors, including smoothing the use of the throttle and taking into account factors such as cat converter warming. ETC is also a technology with major implications for modified performance cars, where sudden changes in torque development (eg an engine coming on cam or boost suddenly rising) can be compensated for, and where throttle response can be tailored to the driving conditions. Take it from us - the first programmable management system with mappable ETC will be a killa. After all, it will be able to include traction control, cruise control and turbo anti-lag at no extra cost....
Anyway, here we take a look at the second-generation Delphi ETC system - one of the most sophisticated OEM throttle-by-wire systems in use in today.
Making it Smarter
A key capability of the latest Delphi ETC is that "it provides new capacity to modify the powertain responsiveness to match driver desires and to compensate for external conditions [thus] providing a more consistent response." Here's how it does that.
1. Driver-Selectable Throttle Response
The Delphi ETC features driver-selectable pedal-to-throttle gain (aka adjustable tip-in). This means it's possible for the driver to change the rate that the throttle opens for any given accelerator input.
There are three tip-in modes:
Normal is the default setting that delivers typical accelerator response under normal driving conditions (progressive and controllable). Manual selection of Power mode sensitises the accelerator pedal to give enhanced performance (sharp throttle response and rapid torque rise). Under slippery conditions, Winter mode maintains maximum driver control by giving a conservative rate of throttle opening. This reduces the chance of the car's driven wheels breaking traction and sliding. Note that adjustable tip-in modes (though in a Bosch system rather than Delphi) come standard on some recent BMW high-performance models.
2. Altitude Compensation
The second-generation Delphi ETC also incorporates an altitude compensated pedal-to-throttle relationship. Driving at high altitude, the air being inducted into an engine is less dense. This reduces the available engine torque and throttle response.
To counteract this loss of performance, the Delphi system automatically multiplies the throttle opening as a function of barometric pressure. This means that the vehicle response remains similar to when it's operating at sea level, and wide-open throttle (WOT) is attained at a smaller accelerator pedal travel. Note that the aforementioned adjustable tip-in function can still operate up to WOT.
The second-generation Delphi ETC is primarily designed for high volume applications. Instead of networking a separate ETC computer to the vehicle's ECU, the current generation sees all processing performed in one combined module. Inside the module are two separate processors - the main processor and the checking processor. The relationship between the two processors is explained under the safety section in the main text.
The main processor accepts inputs from all normal EFI sensors (such as coolant temperature and/or manifold pressure). It also receives inputs from the adjustable tip-in switch, cruise control switch, brake switches, accelerator position sensors and throttle position sensors. Once the combined engine management and ETC processor have made their relevant calculations, outputs are sent to the fuel injectors, ignition system and, of course, the electronic throttle actuator. Meanwhile, the checking processor continually monitors parallel inputs from the brake switches, accelerator sensors and throttle sensors. It also checks the operation of the main processor.
3. Smooth RPM and Road Speed Limiting
The commonly employed fuel-cut method of engine and vehicle speed limiting is very crude. The sudden variation in engine torque output (often at high speed) induces a jarring motion that can unsettle chassis balance. In addition, severe jarring increases wear on the engine and driveline, and accelerated catalytic converter deterioration can also occur (though rapid rises in temperature).
By replacing a fuel-cut system with ETC, it's possible to reduce the savageness of a rev or speed limiter. With ETC set to strangle intake airflow at the critical engine or road speed, revs will fall while the engine otherwise continues to operate normally. This smooth reduction in engine rpm gives a significantly smoother rpm and/or speed limiter transition.
Through its rev limiting function, the Delphi ETC can also protect automatic transmissions from abuse. When the transmission is in neutral, the system enforces a low engine rpm limit by maintaining a near-closed throttle position. This eliminates the possibility of transmission shock if the driver (for whatever reason) attempts to hold the engine in a big free-rev and then slide the selector into gear.
4. Tip-In Bump Elimination
Cars equipped with a manual gearbox (where there's no torque converter to cushion any rotational speed difference between the engine and driveline) can suffer from an unfavourable driveline bump (otherwise known as driveline lash). This occurs when the driver suddenly goes from a trailing throttle to a large throttle opening. The accompanying under-car clunks are caused by the rapid load change in the driveline and the forces that are transmitted through engine, gearbox and differential mountings.
The Delphi system eliminates driveline bump that occurs during this throttling transition. When the driver increases the accelerator position, the throttle is allowed to open to an angle calculated as TP1. TP1 is determined based on rpm and gear position. Once the throttle has reached this angle, a predetermined ramping rate slows the throttle action. The opening rate remains slowed until another predetermined throttle position - known as TP2 - is reached. TP2 is the point where the engine and driveline have been gently loaded against their mounts and there is no further risk of experiencing bump. After the ramping rate is disabled at TP2, a lag filter is used to smooth out the transition to the driver's specified throttle position. This whole process can be completed in milliseconds.
5. Compensation for Catalyst Warm Up Strategies
When a car's catalytic converter is detected as being below its peak operating temperature, car manufacturers often employ spark retard and fuel enleanment to heat it. During these periods, torque and throttle response are slightly reduced - though engineers aim to keep this impediment small enough to go unnoticed by the average driver.
With Delphi's new ETC, however, there's the facility during cat converter warm-up to provide a slightly larger butterfly angle for any given throttle application. This helps to offset the negative driving effects of spark retard and fuel enleanment strategies.
In-Built Safety and Active System Monitoring
For obvious reasons, electronic throttle control requires failsafe operation. The second-generation Delphi system achieves this by continuously looking to detect operating problems and then responding accordingly.
One of this system's primary safety features is its sensor and switch redundancy. The Delphi uses two accelerator sensors (a third is optional), two throttle sensors and two brake pedal switch inputs. This doubling-up ensures the ETC system can remain operational (though perhaps in a reduced capacity) if one sensor goes faulty. Furthermore, the dedicated checking processor continually monitors the integrity of each sensor and switch. Unusual sensor signals are eliminated from main processor computation. Note that the Delphi system's throttle sensors, pedal sensors and brake switches can be supplied with additional voltage reference and ground wires to further improve dependability.
The throttle actuator (which is typically a DC motor driven by a bi-directional pulse-width modulated input signal) is also subject to testing by the checking processor. This ensures the spring-loaded throttle butterfly returns to its default position when un-powered, and that the throttle actuator is reacting appropriately to its commands. Interestingly, a "rationality diagnostic" compares data from the vehicle's AFM and/or MAP sensor against the main processor's throttle position estimated airflow. If a conflict exists, the system is deemed to be exhibiting a fault.
As mentioned, the second-generation Delphi ETC incorporates two processors in one module. The main processor takes care of ETC and engine management duties (such as fuel and ignition), while the checking processor reads all ETC sensor and switch signals. The checking processor also performs self-diagnosis and checks the main processor via a serial link. If a fault is detected, the checking processor communicates with the main processor and a reactive strategy is employed.
It's important that ETC reacts to fault detection with the correct countermeasure. Depending on the severity of the fault, the Delphi system may revert to one of the following four operating modes.
1. Limit Performance Mode
This safety mode will operate "when a reduction in the reliability of determining driver intent has been detected or when the ability to create high levels of engine power is impaired". Most typically it is caused by loss of accelerator pedal redundancy (meaning there's only one operational sensor signal) or there's a processor fault. Once tripped, Limited Performance Mode gives dulled throttle response, reduced engine torque at all throttle positions and brake pedal applications sets the engine to idle. It is still possible to continue driving.
2. Forced Idle Mode
This mode will engage when the system cannot sense reliable information on driver intent. A complete failure of all pedal sensors or a processor failure will cause this condition. As you might've guessed, all that the engine can do in Forced Idle Mode is idle. It will not respond to accelerator applications. The primary aim of this mode is simply to maintain power for vehicle heating, cooling and lighting during adverse weather.
3. Power Management Mode
The only time this mode will activate is when the system is unable to control engine power via the throttle. In this instance throttle actuation is disabled, allowing the butterfly to spring back to its default position. It is still possible to continue driving (albeit slowly) thanks to the main processor's on-going control over fuel and ignition timing.
4. Engine Shutdown Mode
This is the most severe action that the main processor can take. If unable to control algorithms or engine power, the Delphi system disables fuel, ignition and throttle outputs. The engine is totally shut down. The most likely cause of this extreme condition is a processor fault or the inability of the throttle and/or intake system to control airflow.
Do You Say
The term fly-by-wire stems from the area where all 'by-wire' systems were developed - aeronautics. Fly-by-wire means to fly an aircraft using an electronic interface. Therefore, "fly-by-wire throttle" has no place when you're talking about a car (not unless the car is so fast that it can literally fly!). Throttle-by-wire is the technically correct name to use.
Car manufacturers can benefit from modern ETC systems in a number of ways. One is the increasingly common ability of today's ETC systems to use internal torque modelling of any given engine. This torque estimation feature enables elaborate accelerator-to-throttle mapping (to give good bottom-end response in a small capacity turbocharged engine, for example) and is ideal for linking to traction and stability control systems.
Rather than having to equip a vehicle with separate throttle, cruise and engine idle control arrangements, ETC can incorporate it all into one package. From a manufacturer's point of view, fewer components equates to improved sourcing, pricing, application, vehicle mass and warranty issues. Diagnostics and fault response are also enhanced in the latest ETC systems.
As outlined, modern ETC aims to deliver consistent and smooth engine response. It can be adjusted to suit driver preference and road conditions and it automatically compensates for changes in altitude and catalytic converter warm-up strategies. The Delphi ETC can also reduce driveline bump and the harshness of rpm and speed limiting. Automatic transmission vehicles can be protected from reckless high-rpm shifts from Neutral into Drive.
Exhaust emissions can be significantly reduced with the introduction of ETC. Controlling the rate of airflow change through the engine aids in maintaining a near-optimal transient air-fuel ratio. An example where ETC performs exceptionally well is where a driver lifts the accelerator pedal quickly. This action usually generates a large hydrocarbon (HC) spike at the tailpipe. However, electronically tailoring the throttle butterfly close rate maintains a more stable combustion process - giving les total emissions. Note that Delphi claim their second-generation ETC system can reduce tailpipe HC and NOx emissions by an average of 15%.
As mentioned, Delphi states that their second-generation system is designed for high volume applications where ETC is standard fare. It presently comes fitted on several V6 SUVs and smaller cars such as the Opel Vectra 1.6.
For more information: Delphi systems at www.delphiauto.com and AutoSpeed's article on the Bosch ME Motronic system (which incorporates ETC) at "The Bosch ME-Motronic System, Part 1".