Pressure Sensors: The Design Engineers' Guide

Pressure sensors: 10 innovative automotive applications

Driving would be an entirely different experience without all the pressure sensors used throughout the modern vehicle, helping to manage everything from braking to electric windows, exhaust emissions to power steering.

In fact, most of the critical systems in a vehicle rely on pressure sensors to measure and monitor key parameters, which has become central factor in making our roads safer, lowering pollution and improving our driving experience.

But how exactly do pressure sensors enable better vehicles, and what do manufacturers need to know in order to do that?
 

1. Detecting early faults in hydraulic brakes

That easy braking sensation you’re used to and the responsiveness of the pedal beneath your foot is down to a complex blend of components, including pressure sensors. In-car systems detect the pressure you’re applying to the pedal then amplify it to make your efforts more effective.

These systems use an absolute pressure sensor to monitor the vacuum maintained in two separate chambers inside the brake servo (see diagram to the right).

Under normal operating conditions, when the brake pedal is depressed it allows atmospheric pressure to flow into one of the chambers. This increases the pressure on a diaphragm, which, in turn, increases the effort applied to the master cylinder. When the brake pedal is released the vacuum is restored using a vacuum source, which may be via a dedicated pump or drawn from the manifold.

A fault condition arises if the vacuum in one or both chambers cannot be maintained or restored. An absolute pressure sensor is used to monitor the pressure in the chambers and alert the driver or engine management system if the pressure inside the chambers is not low enough to be effective.

Without a way of measuring the pressure inside the chambers, the system could fail without the driver knowing and result in a sudden loss of braking efficacy, just when it’s needed most.

Manufactures are using Manifold Absolute Pressure (MAP) sensors in this kind of application, which can be supplied in surface-mount packages and able to measure pressures in the range of 10 to 150 kPa (kilopascal) with an accuracy of 1% across the entire range.
 

2. Optimising the fuel mix to match the air pressure

Making internal combustion engines as efficient as possible has much to do with getting the fuel mixture just right for the prevailing conditions. This includes the actual and desired speed, of course, but also includes making adjustments for the current engine speed, and the engine and manifold temperature.

It isn’t just the air temperature that needs to be measured though; the air pressure is also an important factor when adjusting the fuel mixture and ignition timing. Here, absolute pressure sensors are used to provide the engine management system (EMS) with the information it needs.

The sensors are used to measure the pressure inside the manifold and, because air is drawn in from the surrounding area, the outside air pressure too. Barometric air pressure can have a significant influence on fuel mixture, so by measuring it and compensating for changes, the EMS can tune the engine for optimum efficiency, whether the car is at sea level or 20,000 feet above it.

MAP sensors are used here, too, but in this case they need to be able to measure pressures as high as 400 kPA.
 

3. Cleaning exhaust filters automatically

Diesel fuel is one of the most common forms of fuel for vehicles, especially large haulage, construction and agricultural vehicles, and pressure sensors are vital in making diesel engines as clean as possible.

Particulate filters inside the engine are used to capture the soot and other particles present in the exhaust gas before it can escape into the atmosphere. The filters then need cleaning, which is done by burning off the particulates.

This can either be achieved using an active system which heats the filter to a temperature where the soot combusts, or a passive system using a catalyst.

In the active system (see diagram below), pressure sensors are used to measure the exhaust gas pressure. The cleaning process is triggered when pressure across the diesel particulate filter (DPF) reaches a threshold. This can be measured by using two absolute pressure sensors or a differential pressure sensor.

4. Ensuring the catalytic converter is sealed

In a passive system, particulates in exhaust gases are destroyed using a catalytic converter. In this case a pressure sensor is used to make sure the system can work efficiently even at low engine temperatures.

The catalytic converter needs to get up to temperature quickly in order to work efficiently. Typically, it needs to reach in excess of 300°C but when the engine is cold so too is the catalytic converter. Feeding air into the exhaust manifold triggers an exothermic process, which helps raise the temperature of the catalytic converter.

Once at temperature, the pump for the secondary air valve is switched off and the system is sealed with a valve. Using an absolute pressure sensor positioned between the pump and the valve provides the necessary assurance that the valve is closed properly and the rest of the system is protected from harmful exhaust gases.
 

5. Monitoring exhaust recirculation

Automotive manufacturers are under pressure to bring down overall engine emissions, and one tool in the box is to recirculate part of the exhaust gas.

Effective in both gasoline and diesel engines, the technique lowers the temperature in the combustion chamber, which has the effect of reducing the amount of Nitrogen Oxide generated and emitted.

Controlling the engine gas recirculation (EGR) process involves using an absolute pressure sensor to monitor the pressure at the valve. Without that control the system could become unstable and result in too much or too little gas recirculation.

Sensor manufacturers are constantly striving to improve their processes to deliver pressure sensors that are better able to withstand the harsh environments present in this class of application.
 

6. Checking the pressure of critical fluids

Perhaps the most common use for an electronic pressure sensor is to measure the pressure of the vehicle’s critical fluids such as engine oil, gearbox and transmission oil, and the hydraulic oil in the braking system, cooling system and fuel systems.

An electronic pressure sensor will have part of its structure exposed to the fluid being measured, so they need to be robust and resilient. Typically, it will use the piezoresistive effect, which detects the change in resistance of a material resulting from deflection caused by the pressure exerted by the fluid.

Pressure sensors targeting this application space will typically be able to withstand extreme environments and be sealed to IP 6k 9k (dust tight, high-pressure steam/jet cleaning) and be able to measure pressures from 0 bar to as much as 600 bar across an operating temperature range of -40 to +125 °C.
 

7. Stopping doors from catching your fingers

Electric door closing on cars is a great innovation but if you (or someone smaller) gets between the door and the frame at the wrong time, trouble can result – but pressure sensors are there to help.

Using relative pressure sensors connected to a sealed hose and mounted around the edge of the doorframe, any obstruction can be detected quickly and reliably.

Any compression of the hose causes the pressure inside to rise, which is instantly picked up by the relative pressure sensor and conveyed to the vehicle’s safety system. If the door is electrically activated, it will stop closing; the same technique works for windows too.

Sensors designed for this emerging application are typically compliant with the PSI5 (Peripheral Sensor Interface 5) protocol, which was originally developed as a reliable interface between airbag sensors and ECUs, and uses a twisted-pair that carries both power and data. Pressure sensors designed for this safety-critical application operate over a range of around 50 to 110 kPa.
 

8. Detecting leaking vapours

Part of the responsibility of car manufacturers is to keep the environment free from potentially harmful vapours produced combustion engines.

New petrol vehicles now include a system that prevents these vapours from escaping the sealed fuel system, normally by routing the vapours to an evaporative system, which contains activated carbon. Air is mixed with the vapours so they can be safely burned up by the engine. Known as evaporative emission control (EVAP) systems, they are strictly tested.

An absolute pressure sensor monitors the integrity of the sealed system at all times, alerting the car (and driver) if a leak occurs. Without the pressure sensor monitoring the system, vapours could escape in the event of a breach, not only releasing harmful vapours into the atmosphere but also putting the manufacturer at risk of prosecution for not complying with regional regulations.

The barometric sensor will likely be located inside the fuel tank, and may provide either an analog or digital output, measuring a pressure range of around 40 to 115 kPa with an accuracy of 1.5 kPa or better.
 

9. Activating airbags faster

Car manufacturers are continually innovating to improve passenger safety. Modern cars don’t just have the airbags in the dashboard; they have them all around the interior, including airbags in the door to protect occupants in the event of a side impact.

The sudden pressure change that occurs in the door cavity during a side impact can be detected using a relative pressure sensor, often much faster than using other techniques. Using the right kind of sensor in this application tells the car’s safety system to deploy the airbag within a few hundredths of a second and normally much quicker than a front airbag system operates. This is necessary because the proximity of the door to the passenger reduces the available reaction time significantly compared to a dashboard airbag system. And in this context, milliseconds count.
 

10. Releasing pedestrian airbags

In the unfortunate event that a car hits a pedestrian, a recent innovation uses pressure sensors to deploy a safety mechanism (an active bonnet system) which is designed to reduce the impact to the pedestrian if they land on the bonnet.

By putting relative pressure sensors in the front bumper of a car, any deformation to the bumper can be detected immediately. If this happens, the car’s safety system can activate a compressed air reserve in the engine bay, which pushes the bonnet up and towards the front of the car.

The elevated bonnet (as shown right) creates a barrier between the pedestrian and the harder components of the engine, thus reducing the potential severity of the impact. 

Some cars also deploy an airbag from the engine bay that covers the windscreen to further protect the pedestrian.

Pressure sensors play a crucial role in all of these innovations, making for a cleaner, smoother, and safer ride.

The automotive market is now one of the largest markets for pressure sensors and is likely to remain so due to the huge variety of ways they’re used.

From a host of safety features to reducing pollution and optimising engine efficiency, pressure sensors are central to the modern motoring experience. Without them, we could easily still be starting our cars with a crank, changing gears with three sticks and hoping we don’t need to stop too quickly!
 

What else are pressure sensors used for?

Find out more in our articles on applications in automotive, building automation, consumer and wearables, medical and industrial applications.

Pressure sensors: 8 life-saving medical applications.

Pressure sensors: 8 building automation applications.

Pressure sensors: 11 life-enhancing consumer applications.

Pressure sensors: 3 applications enabling smart factories in Industry 4.0.

Looking for more on pressure sensor technology? Check out the further chapters of this guide below, or if you're pressed for time you can download it in a PDF format here.

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Pressure Sensors Chapter 1 GBL

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Chapter 1

How pressure sensors work

An introduction to pressure sensors covering the different types, how they work, their function, construction, and what to consider in your design choices.

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Chapter 6

The core pressure sensor technologies

What’s the difference between the different pressure sensor technologies? And how do you know which one to use?

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Chapter 3

The different types of pressure sensors

Discover how pressure sensors vary according to the type of pressure measurement, sensing principles, output signal, media, MEMS technology, mounting and more.

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Chapter 7

Pressure sensors for different media types

An in-depth guide to pressure sensors for different media types. Learn about the technology, applications, different options, their specifications and their limitations.

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Chapter 4

Pressure sensor output signals

Sensors, transducers, or transmitters? The right selection is important for your application. So what's the difference and how do you choose between them?

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Chapter 8

Pressure sensing in harsh environments

An in-depth guide to pressure sensors for harsh environments - designing for extreme temperatures, high pressure, and corrosive and dynamic environments.

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Chapter 5

Types of pressure measurement

What’s the difference between absolute, gauge and differential pressure sensors? And how do you know which one to use?

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Chapter 9

Understanding specifications

Explore the datasheet and the different factors affecting the accuracy of pressure sensor readings. Discover how to make the right choice for your application.

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