The Pros And Cons Of Hybrid Engines

Introduction

The hybrid engine is, essentially, an engine that combines the principles of combustion and electric engines which allows a more efficient power output, depending on the scenario presented to it. They are used in a wide variety of modern machines, in the present time mostly within cars; ranging from an everyday family car, to a high-performance race car. The hybrid engine has benefits in both these given vehicles, such as faster acceleration and better miles per gallon of fuel used (mpg). A key feature to the hybrid engine is its ability to recharge its own battery. It does so through ‘regenerative braking’, in which a small motor is spun upon deceleration, which converts the kinetic energy from the rotating axles into electrical energy. This energy is stored in the battery for later use.

Types of Braking

Regenerative braking can only be used in vehicles that use electric motors, such as fully electric vehicles and hybrid engine vehicles. The electric motor, when run in the ‘forwards’ directions will perform work by converting electrical energy into mechanical energy and using this to drive the wheels. But, when it spins in ‘reverse’, it acts as a temporary electric generator which converts the mechanical energy from the spinning axels into electrical energy to be stored in the car’s battery. The mechanical energy that drives the temporary electric generator comes from the momentum of the car. As the mechanical energy is drawn, the car will begin to slow down as a result of losing momentum. So the engine, when running in ‘reverse’, acts as a brake, hence why we call it “Regenerative braking”.

In addition to an electric motor, most hybrid or fully electric cars will also be equipped with standard friction-based brakes. These are used when the regenerative brakes will not provide enough braking power to slow the car. The friction brakes are much simpler, having a pad on the disc of the tyres that converts the mechanical energy of the wheels into thermal energy when the brakes are applied. This thermal energy is considered wasted energy as it is just expelled into the environment.

As both types of braking are available, the type of braking used is dependent on the application of the engine. Typically, these parameters are pre-set in the car’s built-in computer as a function of pressure applied to the brake pedal. This means that, at a certain speed, the regenerative braking will no longer be used, only the friction brakes. However, it can be changed manually by the driver to optimise the vehicle, although this is only practiced by drivers of high-performance sports cars or race cars.

The environment in which the car is driven also affects the type of braking used as well as the efficiency of the car. For a standard passenger car, such as the Toyota Yaris Hybrid, it is far more efficient when driven in a city due to the stop start nature of city roads. When pulling away, the electric power is primarily used to drive the wheels, if no petrol is needed the car will pull away solely on electrical power. And then when the car stops again, electrical energy is restored to the batteries by regenerative braking. And due to the low speeds a car would typically drive at in a busy city, the friction brakes are rarely used meaning more use of regenerative braking.

In an ideal world, this process would be 100% efficient, with no energy losses in the form of heat or sound. In the real world, however, this process in slightly less than 50% efficient, but has the potential to reduce fuel consumption by up to 25%.

High Performance Hybrids

If we now move to the other end of the spectrum, we get high performance race cars. I will be mostly focusing on the Porsche 919 Hybrid Evo. In 2017, the Porsche was taken to Belgium to set a lap time on the Grand Prix circuit. The fastest lap recorded by the team on that day was 1:41.770 minutes, which, incredibly, was 0.738 seconds faster than previous course record (set by Lewis Hamilton in the Mercedes F1 WO7).

The Porsche’s power and energy recovery systems were optimised for acceleration and speed, with its chassis and wheels perfected for handling and downforce. Another incredible feature of the 919 Evo is that it produces more downforce than an F1 car. Admittedly, a few of the technical regulations from the WEC and FIA were broken, but only marginally. Stephen Mitas, the chief race engineer for the team, said:

“Actually, even the Evo version doesn’t fully exploit the technical potential [of the Porsche 919 Evo Hybrid]. This time we were not limited by regulations, but resources.”

The Porsche is built with two different energy recovery systems, one being the break energy from the front axle, very similar to that on a standard hybrid car. The second method is known as Exhaust ERS (energy recovery system) and it uses the heat from the gases expelled from the exhaust to generate electrical energy. The combined electrical energy produced by the two energy recovery systems is stored in a lithium battery, that is liquid cooled, until needed.

Interestingly, the 2 axels on the car are driven by two different power sources. The rear set of wheels are driven by the combustion engine only. This provides them with a high and consistent power input, allowing high speeds to be maintained whilst still providing huge acceleration. The front two wheels are powered only by the electric engine and are only powered during acceleration. This gives the car a temporary 4-wheel drive, which massively improves acceleration and hence allows a faster lap.

The regenerative brakes are only generating when the car is decelerating into a corner. This does provide a high input of electrical energy, but it is not a constant source. However, the exhaust energy recovery system is constantly charging the ion battery with electrical energy generated by the heat expelled. This constant energy recuperation is vital, as it is all used accelerating out of each corner or bend in the circuit. So, having a constant supply simply means more energy can be used to drive the front axle upon acceleration.

To put into perspective how much energy can be recuperated, the Porsche Evo, during its record braking lap of the Belgian circuit, used 8.49 megajoules of energy in the front axel alone. If we take into account the efficiency of the engine (normally between 70% - 90% for a high-performance race car) the actual energy recovered by the car can be calculated as somewhere between 9.43 MJ and 12.27 MJ. This gives an incredible 120.6 kW of power generated by the car on average over the lap.

The driving force of the electric engine is nothing to laugh about either. The combustion engine provided the Porsche’s rear axle with roughly 700 bhp, while the hybrid system provided the front axle with just over 440 bhp (bhp is brake horse power). This means around 40% of the 919 Evo’s power comes from the electric hybrid engine.

Electric Engines

In both of the above cases, the electric engine is fully sustainable. It only charges from wasted energy the engine would otherwise have considered useless. It does, however, depend on having a petrol run engine to start with. Without the energy supplied by combustion of fossil fuels, there would be no wasted energy to recover and convert to electrical energy. In this sense, hybrid engines are just a more efficient way to use the energy provided by a combustion engine.

There is a solution to this, an engine that is powered solely by electrical energy. They have much larger batteries for higher energy storage capacity and they can be charged via regenerative braking, sometimes by exhaust energy recovery systems, and by simply plugging them into to a power supply.

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This power supply can come from the mains electricity in your home. An adapter and charging devise are plugged into the socket and connects directly to the cars battery and will charge it while the car is not in use. Alternatively, most major cities and companies now offer charging stations. These allow you to park your car there during the day and charge it with the power supplied. This is supplied by either the local council or the company who owns the site. In a lost of cities, this is a free service, meaning it costs absolutely nothing to park and charge your electric car whilst you are, let’s say, at work for the day. This is one of the government’s efforts to increase the popularity of electric cars in major cities.

The government is also pushing hybrid cars to be more commonly driven around the country. This is for several reasons, but again they mostly affect city driving. The reasons include: quieter travel for less noise pollution, cleaner travel with less greenhouse gas emotions by the cars, and less fossil fuel consumption in day to day travels (i.e less petrol used).

Energy Sources and Costs

Electric powered cars are, however, only a way to move the problem away from the cities at the moment. The electricity from mains power, the way the cars are charged, is still produced unsustainably. Over 75% of the electricity supplied to the UK is via coal and gas-powered stations. Only 5.5% is renewable energy sources, including tidal, solar and wind farms. The rest is made up from nuclear power. All this means is that 95% of the electricity supplied to the cars by the mains is still unsustainable.

The fossil fuels and gasses are still combusted to produce electrical energy, just at power stations instead of in individual cars. The greenhouse gasses are still being produced but due to the larger scale it is naturally more efficient energy production than each cars combustion engine. It’s cheaper too, as it costs less to continuously run a power station then to stop and start an engine. Also, with 5.5% of the electricity genuinely being made from renewable sources, it is more renewable than sole combustion alone.

Cost is a big part of everyone’s decision making, and it is one of the reasons the government are helping pay for electric and hybrid car charging. They are typically more expensive than non-hybrid equivalents, but they are much healthier for the local environment. The initial cost will slowly be made up for by the savings on petrol or diesel, so the car is an investment in the very long term. But the initial purchase is still very expensive, and if the city or location the you park your car in everyday doesn’t provide free charging, or charging at all, then it isn’t worth the money.

With this in mind, hybrid and electric cars are generally a good investment if you have access to local charging, and especially if the charging is free. It is typically easier the wealthier members of society to purchase hybrid and electric cars as they are, as previously mentioned, considerably more expensive.

This could potentially raise a class issue, as people with less money won’t have the access to this almost free electricity as they wouldn’t have had the extra money needed for the more expensive hybrid or electric engines. Currently, this is not an issue, but with the government trying to ban the purchase of new petrol and diesel cars by 2032, it could become a significant problem. It is likely, however, that as the development of hybrid engines and cars continues, they will start to become significantly less costly.

Hybrid and Electric Car Models

The rate of development is continuing for hybrid engines, every year they become more efficient, cheaper to produce, and more practical for the general public. The shear number of companies and organisations who are working to improve the hybrid and electric engines is enough to almost guarantee results, with companies like Toyota, Volkswagen, and Mercedes all involved.

Unfortunately, most of these companies are racing each other to the solution and will very rarely combine ideas and expertise. While this does provide a level of competition and increase the pressure to develop more efficient and practical engines, it also hinders development by not using a wider pool of knowledge.

To illustrate this point, 3 major companies are all striving for greener travel in incredibly different ways and are having a hard time to justify the cost of their products when they don’t offer a particularly big benefit.

Volkswagen is working on a ‘Hover Car’ that was based off the Japanese Maglev train. The idea behind this is that instead of the normal reaction force being provided by tyres, or train wheels, it is provided by electromagnetic repulsion. This means the car is held above the road by a magnetic field, which would massively reduce drag forces by almost completely negating friction. This increases the efficiency of a car massively as so much of the output energy is dissipated due to air resistance. But naturally, the car can only support a maximum of two passengers as there isn’t a strong enough natural electric field within the Earth to provide more force, at least not using current day electromagnets.

Another company, known as EVX, is working on a very different method of energy saving. Their ‘Immortus Solar Car’ is built with 7 square metres (75 sq ft) of solar panels lining the roof and bonnet, as seen in image 1 above. One representative stated that in sunny conditions “the inbuilt solar panels alone will let you drive at more than 60 km/h (37 mph) for an unlimited distance.” The Immortus is also incredibly light and has a very aerodynamic design. The incredibly rounded roof and sharp nose edges, combined with the covered wheels, seriously reduce air resistance, but at an expensive price.

Unfortunately, the Immortus only has a 10kWh lithium ion battery, which means it has a much shorter range at higher speeds. If the Immortus team were to combine some ideas with Tesla, who are producing a car with a large 85kWh battery, then the range would be immense, and the car would be far more practical and reliable, even on less sunny days. The Tesla runs from power supplied to the battery from mains whilst charging so it needs the large battery to travel any long distance.

Of these 3 extremely different approaches to designing a new generation of electric and hybrid cars, only the Tesla is currently available and functional. This is mostly due to the simplicity of the design. The other 2 cars use a more unorthodox approach to the problem, and if they can be successfully designed and manufactured, they will provide a huge help to the common goal of producing sustainable transport.

But as we know, all these companies are in rivalry to each other and are incredibly protective over their ideas. If they simply combined designs they could have a functioning prototype far sooner, and it would be much more effective. Combining all 3 of the examples above would create an unbelievably lightweight vehicle with little to no drag or air resistance with a near infinite range, and no fossil fuels or greenhouse gasses to be heard of.

Conclusion

It is no easy feat to completely eradicate the combustion of fossil fuels from transport. It is considered impossible with todays technology to have a cargo plane take off and sustain flight with electric power alone. But with cars, they require much less power and it is a target set by numerous countries to remove fossil fuels from the roads. We are running out of petrol, natural gasses and coal so the development of alternative power is urgent. We have started to see functioning electric cars in recent years and this progress has continued with more funding, time and resources. The near future promises hybrid engines in cars, no more combustion only, and 100% electric engines soon after. With only around 30 years of fossil fuels left we need to see this become a reality.

Summary

This essay provides a clear exploration of the pros and cons of hybrid engines, including a comparison to fully electric engines. It focuses on the different types of braking used by hybrid cars as well as the mechanisms of energy storage within various car battery types. From my evaluation of different car models which use a hybrid engine, Toyota, EVX, Volkswagen. I was able to analyse how these hybrid models may adapt to the needs of the future. Overall, I can conclude that the hybrid engine has not yet been fully utilised and will become an essential component of future transport.