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Tuesday, November 17, 2015

Racing Engineering | Technology

Ever since cables first connected accelerator pedal to carburetors, drivers have been searching for better throttle response - the connection between input at the pedal and power delivered to the wheels. Electronically controlled throttle offers an interesting solution and may even change what we consider the throttle’s job to be. There are two important maps for the driver: the Driver Pedal Map and the Driver Torque Demand Map.

The Pedal Map and the Torque Demand Map are two maps that allow the driver to manage the engine
drivability through the redefinition of torque delivery at wheel and in turn, to improve the laptime performance. When we are talking about response or feel, we are generally referring to how hard the car is
accelerating in relation to throttle position. If we open the throttle more, we want to accelerate more; less throttle, less acceleration. If the car had a perfectly flat torque curve at every rpm and every throttle opening, the relationship between throttle opening and acceleration would be nicely linear. But that is not the case. As you accelerate out of a corner, the amount of torque the engine generates at a given throttle opening changes as rpm scales through the torque curve’s peaks and valleys. In many cases the relationship is anything but linear, and it’s possible to even end up closing the throttle slightly on a corner exit while the engine climbs out of a dip in the powerband in order to keep torque constant. With throttle butterflies controlled by electronics becoming more common, it’s possible to change things so that torque output - what we feel - is more directly related to throttle position. As you exit a corner and open the throttle, torque goes up proportionately. Close the throttle, and torque goes down proportionately. There is an additional benefit in this scenario: not only will response be improved, but also the engine can be tuned more for peak power rather than a smooth power curve - the electronics will hide any resulting peaks and valleys. In short, the driver torque map represents the torque requested by the driver as a function of engine speed and accelerator pedal position.

There are few required maps in engine mapping:

1. The engine torque map engine torque map is a theoretical model of the engine. It represents the torque delivered by the engine as a function of engine speed and engine throttle position. In the ECU the engine torque map is used to position the engine throttles to match the drivers’ torque demand. This is a 2-dimension table with engine speed and throttle as inputs and torque as output. This map is defined point-by-point or by ramps at the test rig with the fired engine and the torque meter.

2. The inverse engine torque map . It is calculated by the engine torque map. It is a 2-dimensions table with torque and engine speed as inputs and throttle as output. It is computer calculated.
3. The driver demand torque map . The driver torque map represents the torque requested by the driver as a function of engine speed and accelerator pedal position. This is also 2-dimension table with engine speed and driver normalized torque as input and engine torque as output. This is a reshaping of the engine torque. This is the core of the drivability because allow the engineers to completely reshape the engine torque in function of the engine speed, within the boundaries of maximum and minimum torque available (which is the torque at off-throttle – engine brake - and at wide-open- throttle).
4. The pedal map . This is a 1-dimension map where the input is the normalized accelerator pedal position (0-100%) and the output is the normalized torque (0-100%). It can be considered as a “gain” on the driver demand torque map.

5. Ignition map . The key thing when tuning an engine in terms of controlling fuel detonation. Determine proper way of fuelling, air-fuel Ratio (AFR), exhaust gas temperature and ignition position.

So, with the generic term “Engine Maps” we refer to a wide set of one-dimension or two-dimension parameter tables loaded into the ECU to control all the engine parameters, that are, at least, throttle opening, injection and ignition timings (duration, phase, etc...). All maps are filled with proper data during hours of engine calibration done at the test rig and/or at track. The torque demand, as requested from the driver by the acceleration pedal, is calculated by the chain composed by the Pedal Map and the Torque Demand Map. The input variable is the accelerator pedal position, given by the drive by wire potentiometer and the output variable will be the throttle position, actuated from the ECU by hydraulic or electric actuators on the engine intake butterflies or barrels.

Electronic throttle control can be accomplished in one of two ways. The first is by using an open-loop, map-style programming of the ECU, much as a manufacturer’s base fuel mapping is modified using a Power Commander or other fuel injection tool. Knowing the torque produced by the engine at any given combination of rpm and throttle position, an electronic butterfly’s programming can be modified to react accordingly, based on what the driver is asking for. The second is to use a closed-loop system, with a torque sensor providing feedback to the ECU. Engine mapping affects ignition timing, fuel mixture, power and torque delivery.

The Pedal Map and the Torque Demand Map are two single most important maps that allow the driver to manage the engine drivability through the redefinition of torque delivery at wheel and in turn, to improve the laptime performance.

The engineers provide a selection of usable pedal maps and torque maps and driver can change and select desired map by using rotary selectors on the steering wheel. In this way, the driver could have the optimal pedal map in function of the corner (different between low speed corners or high speed corners) or have negative slope torque map for low gears and standard map for high gears - all selected automatically. The selector for the pedal map is often labeled as "PEDAL" and the torque map selector is often labeled a "TRQ". Sometimes the two functions can be overlaid on single rotary selector, to save one rotary selector for other uses. It is recommended to always have a WET configuration on the selectors, which can be used when grip is low. Except for some specific exceptions, the engine torque must be controlled by the driver by means of a single accelerator pedal. These exceptions include: downshifts, pit lane speed limiter, anti-stall function and the end of straight limiter strategy. Note that this list is not exhaustive.

Engine torque mappings 
This 2-dimension table map is defined point-by-point or by ramps at the test rig with the fired engine and the torque meter. Sometimes is trimmed on track if the car is equipped with torque meters on the transmission.

Renault Sport provided me with detailed technical explanation on engine torque mappings. Torque maps were a hot topic in July and August 2012 when the FIA issue a directive and made some changes to the way teams were using them during the mid-season. But they are the single most important map for the engineers to use for reference when trying to make sure that the engine is optimized for a given circuit.

A directive from the FIA around the time of the German and Hungarian Grands Prix closed a loophole exploited by Red Bull Racing and its engine partner Renault, to improve gas pressure from the exhaust by lowering the torque curve. As a result the Formula 1 teams are no longer allowed to change the torque maps from weekend to weekend, so the maps used after 2012 Korea GP are the ultimate expression of what an F1 engine can do.

Renault Sport F1 engine engineer David Lamb:
“In it's simplest form, the engine torque map is a theoretical model of the engine. It represents the torque output of the engine for a given engine throttle position and engine speed. In this respect it appears outwardly similar to a driver torque pedal map, the only change being the look-up against engine throttle position instead of the driver’s pedal. However, in reality, the differences are far more complex and wide reaching. From this map, you know for any given speed or throttle position that you should produce a certain amount of engine torque,”

“We then use that reference map to ensure the engine is behaving as it should out on the circuit. We measure the actual engine torque with an on-board sensor, and when you overlay this with the value predicted by the torque map, you shouldn’t notice any large differences. If you have a hesitation or a drivability issue, you will see it clearly because the measured torque will not match the reference torque.”

“The torque map doesn’t change much over the course of a weekend, or between races. Under the new technical directive, issued between the German and Hungarian Grand Prix, you can’t really change the maps that much over a weekend or between races. It’s like a fingerprint of the engine. There will be subtle differences between the teams due their respective air boxes and exhausts, which will slightly change the form of the map. Prior to this directive, we would change the torque map freely to suit the climatic conditions. For example, the engines will produce nearly 10% less torque at Sao Paulo than they will this weekend in Korea due to Sao Paulo’s high altitude. By changing the torque map to the prevailing conditions the engine response will feel the same to the driver across the season. Nowadays we have to request this torque map change from the FIA, and fully justify our reasoning.”

“As well as ensuring the engine behaves as it should, the map is also used to improve the drivability of the car for the driver. When the driver lifts off the pedal the engine can be either fired in four cylinders or fully cut, depending on the level of overrun support he requires. When the driver goes back on the pedal from full ignition cut, you need to inject more fuel than usual to ‘wet’ the engine. Inject too little or too much and you will have a torque deficit from target, which can cause a hesitation and a loss of lap time. The initial torque demand will generally be met with only four cylinders, as you’d rather save a bit of fuel and have four cylinders firing strongly using a more open throttle than have eight coming into life rather weakly with a relatively closed throttle."

“When the torque demand exceeds that which can be met with just four cylinders, the remaining cylinders need to be fired. These will also require ‘wetting’. At this point you also have to close the throttles at a rate which coincides with the final four coming back into life – this is the tricky bit! Get it right and the driver should feel nothing across the transition, just a change in engine pitch. In all cases, the torque map is used in conjunction with other settings to govern both the fuelling requirements and throttle position.”
The engine torque map is used for a multitude of other processes, such as the pit limiter, rev limiter and
downshift control.

“The engine torque map is without doubt one of the most important calibrations in the SECU. It really is the reference point. When the driver lifts of the pedal, it’s the engine torque map that decides by how much we close the throttles. When he goes back on power, it’s the engine torque map that stipulates to what point they open. It all works off that map.”

Pedal mappings
Gas pedal is no longer a simple way of directly moving the throttles on the engine, because the ECU replaced traditional Bowden cable between the pedal and throttle with pedal position sensor and a map. Such maps are now restricted to tire type, so just three maps are allowed: for wet weather, intermediate and dry tires. Previously, different maps could be selected for the race start and other race situations. The regulations also enforce other restrictions on the pedal, for example no detents or other means can be used to aid the driver in holding a specific position, such as holding revs steady at the race start.

5.5 Power unit torque control :

5.5.4 The accelerator pedal shaping map in the ECU may only be linked to the type of the tyres fitted
to the car : one map for use with dry-weather tyres and one map for use with intermediate or wet-weather tyres.

5.5.7 Homologated sensors must be fitted which measure the torque generated at the power unit output shaft and the torques supplied to each driveshaft. These signals must be provided to the FIA data logger.

Detecting the pedal travel has evolved since drive-by-wire was introduced. Initially, linear or rotary sensors between the chassis and pedal were used and, owing to the critical nature of the sensor, these were doubled or even tripled up for redundancy. In recent years, for simpler packaging the linear sensors have been replaced with non-contact sensors, such as Hall effect or induction types, the sensors being built into the pivoting plate and the target magnet being embedded into the base of the pedal.

Changing the pedal map is an easy way to change the sensitivity of the pedal while conserving the characteristic shape of the driver demand torque map. They are one of the main factors which allow matching an engine's feel to a driver's requirements. Role of pedal maps is to ensure the driver always has the power he requires, they can also be used to give the driver a bit of help.

Renault Sport F1 engine engineer David Lamb explains:
“There are essentially two types of pedal map. There’s a conventional one dimensional pedal map, which is basically a representation of a driver’s throttle against the pedal input that is passed across to the engine controller. You can use this to quickly change the feel of the engine to the driver, but it is slightly obsolete now; you might have found it in the sport ten years ago. Some road cars now feature a ‘sporty’ pedal map of exactly this style, with the initial engine response feeling more aggressive to give the impression of it being racier.

The challenges in Formula 1 in terms of throttle mapping are very different from what those car makers face. Since traction control is banned in Formula 1 it’s up to the driver to modulate the throttle. Getting the right amount of engine output to accelerate the car out of the corners without excessive wheel spin (too high slip ratios) is critical for ensuring good lap times while limiting tire wear. To help the drivers, the engineers try to adjust the throttle map to each driver and corner to make it easier for the driver to maximize traction.

“Now when we talk about pedal maps we talk about the torque pedal map, which is a two dimensional map against engine speed and throttle pedal position. For a given pedal position and a given engine speed, you generate an engine torque demand from the driver. It is this demand that gets fed to the engine side of the ECU to deliver the required amount of torque.”

“With driver torque pedal maps, you can have different philosophies. You can have a constant torque map, where regardless of the engine speed you receive the same torque demand for a given throttle pedal position. However, this offers no wheel spin assistance, which can be incorporated with a constant power style pedal map.

“For example, say you’re at 50% pedal on the throttle and at 15,000 RPM, you might get around 200Nm of torque. If you get a bit of wheelspin and the engine speed increases to 16,000 RPM, the torque at the wheels will be reduced as this is a constant power pedal map – power being the product of torque and engine speed. It’s not traction control as it isn’t controlling to a wheel slip target, but instead an open-loop method to try and help wheelspin control. It can be of real benefit when the tires are worn out.”
“You can have an area of constant torque on a pedal map followed by a region of ‘constant power’ decay afterwards. Your torque pedal map could therefore be a mix of this and a constant torque map, depending on the preference of your driver and your car.”

But it isn’t just in the use of the pedal that mapping is important. It is also when the driver is off-throttle that is of almost equal importance.

“The zero percent line is when the driver is completely off the pedal. It is this line that sets the amount of over-run push. This is when the engine continues to turn and produce torque, albeit still slightly negative, under braking. Getting this correct is essential as we use it to reduce rear locking under engine braking . It’s another open-loop system, this time pseudo anti-lock, so as the tires wear out the driver will tend to increase the amount of push during the course of a stint.

“The downsides are that the heat rejection to water and oil will go up, so fluid temperatures will increase, and you’ll also use more fuel to achieve this. It’s quite a pronounced effect: if you plan to use maximum push for the whole race you could end up adding another two kilos to the starting race fuel load.”

There are, of course, limitations that have been placed upon pedal maps so that a version of traction control doesn’t edge its way back into the sport and the FIA is quite strict on how teams use pedal maps, especially on launch procedure. But during the Fridays teams are working incredibly hard to make sure that they hone pedal maps for each corner and for each driver to make sure that they are happy with throttle application and power output.

Ignition map
Ignition mapping is the key thing when tuning an engine in terms of controlling fuel firing and detonation. Determine proper way of fuelling, air-fuel Ratio (AFR), exhaust gas temperature and ignition position. The two things that will kill your engine quicker than anything else are detonation and excessive exhaust gas temperature.

To get your car to first fire up, you need something loaded onto the ECU initially from which you can begin mapping. This is referred to as a base or baseline map. Engine engineer will provide a base map for the first dyno test. The key thing about a base map is that it must be safe. Normally the fuelling will be richer than you need and typically the ignition will be more retarded than you need. Some calls a base map a footballer map as those tend to be rich and retarded too. The base map is not about trying to get the most power or best fuel economy, it's just about getting the engine to start and to drive the car around so it can be mapped further from that starting point to get the desired end result. Engineer will build up gradually on the base map and map the engine during the dyno test and also later during the track test. Normally developed ignition map will look nothing like base map.

Use of engine torque map, driver torque map and pedal mappings are covered by Articles 5.5 of the technical regulations.

5.5 Power unit torque control:

5.5.1 The only means by which the driver may control acceleration torque to the driven wheels is via a single chassis mounted foot (accelerator) pedal.

5.5.2 Designs which allow specific points along the accelerator pedal travel range to be identified by the driver or assist him to hold a position are not permitted.
5.5.3 The minimum and maximum accelerator pedal travel positions must correspond to the minimum and maximum available torque with the currently selected power unit torque map. 

- Except for some specific exceptions, the engine torque must be controlled by the driver. These exceptions include: downshifts, pit lane speed limiter, anti-stall function and the end of straight limiter strategy. Note that this list is not exhaustive.
- The driver may only control the torque by means of a single accelerator pedal.
- At zero percent pedal (off throttle), the torque demand must be less than or equal to zero; at one hundred percent pedal (full throttle), the torque demand must match or exceed the maximum torque output of the engine in its current state (Article 5.5.3).
- There are limits on the shape of the torque demand as a function of pedal position and engine speed (to prevent engine characteristics that could be driver aids). Respecting these restrictions, the torque demand is shaped against throttle position and engine speed to deliver the desired response for the driver and car.

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