eCompressors: a guide to the compressors of the future

The adoption of hybrid and electric vehicles is on the rise, and whilst they might not be appearing in your workshop for a few years yet, it’s good to be prepared.

There are some significant differences to be aware of, one of which is the purpose of the A/C system, which in hybrid and electric cars not only cools the cabin but also the battery which powers the vehicle. “It’s a different approach for workshops who are dealing with A/C in hybrid or electric vehicles. If the A/C fails in a ‘normal’ car, you can still drive and simply open a window. But if the A/C fails in an electric vehicle, the car will not run, as it’s a safety risk: the battery will get too hot,” explains Richard Groot, A/C product specialist at DENSO AM. It’s clear that in future the performance of the A/C system will become increasingly important, and this article aims to provide insight into a crucial component, the eCompressor, plus offer some useful maintenance tips.

electric compressor

What is an eCompressor and how does it work?

Conventional compressors are powered by an engine’s belt drive, however electric vehicles do not have belt-driven engines, therefore a different kind of compressor is required.

DENSO’s eCompressor consists of the following components:

  • Compressor section – contains a scroll type compressor to draw in, compress and discharge refrigerant.
  • Motor – to drive the compressor. A DC brushless type that uses a permanent magnet as a rotor, and a coil for the stator.
  • Inverter – to drive the motor. The inverter converts Direct Current (DC) from the HV battery into Alternating Current (AC) for the motor. Moreover, the A/C ECU inputs compressor rotational speed signals to the inverter via the HV ECU to control the rotational speed of the electric compressor.
  • Oil separator – compressor oil can reduce cooling performance, therefore a separator is used to separate oil from the refrigeration cycle to improve A/C cooling capacity.

The electric motor runs at a high voltage of 200V or greater, and is cooled by a refrigerant, since it generates heat while operating. In order to insulate the electric motor and compressor housing, compressor oil with high insulating properties is used.

DENSO has been mass-producing eCompressors since 2003 and was the first in the world to integrate an inverter with a compressor into one component. DENSO’s latest generation of eCompressors places the inverter in-line with the motor, making the design lighter and slimmer, thus saving more under-the-hood space.

ecompressor drawing

Fuel saving benefits and much more

DENSO’s eCompressor design has a range of benefits for hybrid and electric vehicles:

  • Controlled revolution speed which contributes to the saving of energy. Richard explains, “When you control the electric compressor speed, you also control the energy consumption. The less energy you use to drive the compressor, the more you can use to drive the hybrid or electric vehicle so your vehicle range will increase if the eCompressor offers high performance at low revolutions.”
  • In addition, due to its small size, DENSO’s eCompressor consumes less energy, extending the lithium-ion battery range.
  • The internal electric motor of the eCompressor enables the A/C to continue running, even when the hybrid or electric engine is idling or switched off. “The electric compressor operates independently of the engine. The compressor is always driven by the electric motor, so when the hybrid or electric engine isn’t on, the eCompressor can still operate.” This offers a significant advantage over traditionally fuelled vehicles as it maintains a cool temperature in the cabin.
  • Less noise: the newest design from DENSO is quieter than previous models, whilst retaining the same cooling capacity. This provides a comfortable environment without passengers having to listen to unnecessary noise.

Working with eCompressors

Safety first

Safety is paramount when dealing with high electrical voltage, and it’s important to protect the vehicle and yourself. Richard stresses the importance of being safe, “You have to take some precautions before working on a high voltage system – remember the compressor is driven by the high voltage battery pack. The output is between 200-400 volts or more, you can’t simply change compressors. Before you work, you have to disconnect the battery pack from the vehicle and to do so you have to take precautions.”

Correct oil

It’s important to pay attention to the type of oil used in an eCompressor, as it (usually) differs from the oil used in mechanical driven compressors. ND-Oil 11 is recommended as it is highly insulating therefore protects the electric motor.

“Here it can go wrong, we see examples of workshops where the mechanics are not aware of the different types of oils used and they mix oil and the compressor can fail. If you use the wrong oil, it can cause a short circuit and will destroy the electric motor in the compressor,” describes Richard. If you use a UV-Dye, make sure you use the right type of UV-Dye with SAE approval. Any other (cheap) UV-Dye can create problems with the insulating part of the compressor oil.


Don’t forget your A/C refilling machine. “Usually the A/C refilling machine that most workshops use can only handle one type of oil, so when you service hybrid or electric vehicles, keep this in mind.” There are various different solutions on the market to tackle this issue such as machines which have a ‘flushing’ programme internally, so you can switch up the oil without problems. However Richard recommends having separate tools for hybrid and electric vehicles for added peace of mind.

Maintenance is crucial

The eCompressor is hermetically closed, so there is no shaft seal or potential risk of refrigerant leakage to the outside air. This means that the leak rate of refrigerant is less, compared with a mechanical driven compressor. Still, this doesn’t mean hybrid or electrical vehicles do not need A/C maintenance. Refer to the car manufacturer’s guide for details. “Make sure the refrigerant is at the right level, the problems start when the system runs out of refrigerant or when the refrigerant goes to a critical level as it will usually cause the compressor to fail,” advises Richard.

ecompressor drawing inline inverter

What does the future look like?

As with all technology, eCompressors and the A/C system in hybrid and electric vehicles will only evolve over time, so workshops need to constantly update their knowledge in order to be able to maintain them. Many of the changes will be related to one of the most challenging goals for electric vehicles: the extension of range. Due to high levels of power consumption, the A/C system has a real impact on range and there is room for improvement. Future systems may use an integrated design approach to save on components and optimise lay out, saving space and weight. Holistic and predictive control functions will improve energy efficiency and contribute to range extension.

In Richard’s opinion, “The future will look completely different because the A/C system will become more like a thermal management system; it’s not only about cooling the interior cabin and battery but also about the heating system and efficiency. In 10-15 years’ time it won’t be easy to diagnose problems with thermal energy systems because of the complexity.” All the more reason to stay in touch with the changes and keep an eye on what might be coming next.