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© IFPEN / IFP School 2014

Sustainable MobilityTechnical and environmental challenges for the automotive sector

Week 5 – Session 1 – Hybrids : working principles

Prakash Chandra Dewangan

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W5 – S1 – Hybrids1 p. 1


You must have noticed that emissions, whether it be pollutants or CO2, are the major challenge for

the transportation sector today. Here, we will discuss alternative vehicle propulsion technologies

which are helping us to reach the goals of lower consumption and emissions. This lecture will be

technologies in the domain of hybrid systems.

During the course of these sessions, we will introduce you to the hybrids, discuss the advantages

and disadvantages; then, we will find out how to classify different hybrids and finally, we will

discuss how much is the benefit brought by these technologies in terms of GHG emissions.

Introduction: the advantages of hybrid

Let’s start with this projection of future automotive market till 2050.

What you notice is, today, most of the market is taken up by conventional gasoline and diesel

vehicles while hybrids and electric vehicles are a small fraction of the market. However, in about 15

years, these alternative technologies like hybrids would make almost 50% of the market. So, thismust be important! And we will find out why actually so.

Let’s take an example of a midsize 4 seater conventional car; it runs with a thermal engine; the

cheapest car may cost much less than 10000€ and it runs about 500 km or even more on one fuel

refill. This has been mobility model for last 50 years at least. As we discussed last week, these

thermal engines emit pollutants and CO2, the latter being the major cause of global warming.

On the other hand, we also have Electric Vehicles or EVs; also known as Zero Emission Vehicles

(ZEV) or Battery Electric Vehicles (BEV). They have image of being green car as they emit nopollutant or CO2 locally as they don’t run on fuel but on electricity. Although, how did we produce

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electricity is a different story altogether and we will come back to this towards the end of the

lecture. The two biggest disadvantages for EVs are cost and range; cheapest EV today costs around

25000€ and would run less than 100 km on one charge. So, with the point of view of versatility and

cost, EV is not a winner today.

Hybrid vehicles are the technologies bridging these two extremes trying to capitalize advantages of

the two.

Let’s start understanding the Hybrids! In the context of automotive propulsion, a system with more

than one type of energy onboard the vehicle is called hybrid propulsion system. As we discussed

conventional vehicles run on thermal engine using the fuel either diesel or gasoline or gas as energy

source. And hybrid has alternative propulsion system in addition to the thermal or conventional

powertrain.

Depending on which alternative propulsion system we have, we can have different types of hybrids.

The major classification being: Electrical hybrids which used a mix of electrical and fuel energy. So,

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they have a big enough battery to store electrical energy. One of popular example of electric hybrid

is Toyota Prius.

Alternatively, we can have Hydraulic or pneumatic hybrids which have, apart from thermal engine,

an accumulator to store hydraulic or pneumatic energy. In fact, this storage is like a tank storing

some gas under high pressure. French car manufacturer PSA Peugeot Citroën recently showcased

their pneumatic hybrid called “hybrid air” for car applications.

Apart from above two, we can also have inertial hybrids which has a flywheel to store energy. And

lastly fuel cell based hybrid which consists normally energy in the form of Hydrogen and battery

being the second storage. These two are really a research area as of now and do not have

commercial example.

During the scope of this course, we will be focusing on electrical hybrids because it being a mature

technology with already large number of commercial examples.

Let’s continue to understand hybrid powertrain with the example of electrical hybrid.So, in conventional powertrain, the energy comes from oil wells in the form of fuel which is burned

by the engine and the engine produces torque which goes through the transmission to the wheel.

Transmission in most cases is equivalent to a gearbox.

In the case of electric hybrids, we have this second chain of propulsion which is electrical. So,

electricity produced by different sources charges the battery which drives the electrical motor

producing torque. There could be an additional transmission between electric motor and the

wheels.

Note that there are two different sources of energy, two different storage of energy and two

different systems of propulsion.

Effectively, all this secondary storage and propulsion system are the extra cost of hybridization interms of weight as well as financially.

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Now that we know what hybrid is, let’s take couple of minutes to evaluate advantages and

inconveniences. Biggest advantage of hybrid is lower fuel consumption which reflects directly in

lower CO2 emissions in comparison to their conventional counterparts.

In general, hybrids have lower pollutant emissions and some of them can even run in EV mode for

short distances. However, all these advantages come at a price. Hybrids are costly, can be quite

complex and batteries have low energy density making them heavy and bulky.

Nevertheless, there is significant gain in CO2 which already makes hybrid quite interesting.

Continuing our evaluation of advantages for hybrids, let’s take a real example.

Toyota Auris is a car available in gasoline, diesel as well as gasoline hybrid and it is interesting to

compare official figures on consumption, CO2, price and weight. The numbers are from French

market.

Going from gasoline to Gasoline Hybrid, we gain around 35% in the fuel consumption and CO2

emissions; while diesel counterpart comes somewhere close to gasoline hybrid in fuel

consumption.

This gain for hybrid comes at around 37% higher cost compared to gasoline and with increase of

15% additional weight. This comparison nicely summarizes the tradeoff between fuel consumption

and cost.

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Working Principles

Having convinced ourselves that the hybrids do have practical examples of gain in fuel

consumption, let’s now try to understand how it works and where exactly this extra gain comes

from. Let’s start with the efficiency chain of a conventional car. For a mid-sized diesel car underurban driving conditions, if we take fuel energy to be 100%, engine converts only around 29% of

fuel to useful work while 71% is lost at the engine itself. Continuing further, we see that around 3%

is lost in the transmission and out of 26% energy left after transmission, only 11.4% was used to run

the car. What happened to the energy which wasn’t used to propel the car?

Around 14.6% is wasted in braking the car. Furthermore, out of 71% unused energy at the engine,

59% was the losses in the engine which you discussed in the course of my colleague Maria previous

week. Finally, the rest of 12% energy was lost in idling. Meaning that the engine was ON when car

was not moving, for example a car with engine on at the Red traffic signal.

If we want to improve overall efficiency, one option is to improve the engine technology; or use the

existing engine technology in a better way.Secondly, we can try to recover the losses during braking or idling. In fact, a hybrid system improves

the consumption playing on these two factors: improve efficiency, recover losses.

Let’s go deeper to understand the energy efficiency and why not we start with the engine.

Considering first the engine losses, the graph shows a typical efficiency map for a thermal engine.

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On x-axis we have, engine speed and on ‘y’, we have engine torque. The red-line is the max torque

curve for this engine.

Point to note is that the engine efficiency is not constant everywhere and there are regions of good

efficiencies which is around 40% in this case, and region of bad efficiency especially towards lower

torque.

Under urban driving, engine is used at these blue dots (as shown in the figure in the next page).

Clearly, engine is working on lower efficiency points in this case with a lot of points being at around

10 to 20% efficiency.

So, imagine if we can shift the working points towards this green area of good efficiency.However, as you could remark, now in the green reason we have extra torque than before and in

fact, engine will produce extra energy than needed.

Imagine if there is a high efficiency energy buffer which can store this extra energy and provide it

when needed. This would certainly improve engine efficiency just by better use of existing systems.

Let’s now focus on the energy lost in idling as shown below. Idling is the single blue dot inside the

red circle where engine is on and consuming fuel while it is not producing any effective torque.

Imagine if we can switch off the engine and we could restart it rapidly when needed. This we willcall “stop and start” and it could save around 10% fuel consumption in this example.

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Finally, imagine if we can recover a part of the energy during braking and store that in the energy

buffer too. This braking could be with an electric motor functioning as generator. We can call this

regenerative braking.

Remember that on the left the 100% was the energy coming from fuel and, high efficiency buffer is

a buffer not fuel, meaning it need to be rechargeable with efficiency around 70 to 95%.

In case of electrical hybrids, this buffer is a high efficiency battery.

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We can conclude effectively that if we well design and well manage our hybrid system so that we

can recover idling losses, braking losses and improve engine working efficiency; then, overall, we

could save up to 45% energy in this case. Now we know, where does the gain in consumption of

hybrids come from.

To conclude our discussion on energy and efficiencies for hybrids, let’s see all the energy flows in a

hybrid vehicle; each combination of energy flow will be one possible operating mode. First theconventional mode, in which, the energy flows from fuel to engine and to wheels without use of

electrical system onboard.

Next, a Zero Emission Vehicle or ‘ZEV’ mode when energy stored in battery is used by e-motor to

run the vehicle without using any fuel.

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Next a hybrid mode of operation is Torque Assist mode when both the chains namely thermal and

electrical are providing energy to run the car. In other words, thermal engine is assisted by

electrical powertrain to run the car.

Next mode is regenerative braking when car is braked by the electrical motor and it recovers the

kinetic energy of the car and stores it in the electrochemical energy of the battery.

Finally, battery recharge mode when part of the energy from engine is used to propel the car and

the other part is used to recharge the battery using e-motor as a generator.

You should make link which our previous discussion on efficiency that ZEV mode might be useful

when we need zero emission or when thermal engine, if used, would run in really bad efficiency

zones. Similarly, regenerative braking mode is helpful to recover braking energy. While battery

recharge mode can push engine to work at higher torque points where efficiency is better than thetorque needed to run the car and additional energy produced is used to charge the battery.

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