Manual Transmission, How it works ?

Manual transmission, or simply a gearbox, has been serving automobiles well for many decades. Even today it’s the most popular form of transmission. Globally manual transmission accounts for 52% of market share as per 2013 data. In this video, we’ll give a conceptual introduction on the workings of an actual manual transmission with a reverse gear.

A detailed webpage version of the video is given below.

Why the Transmission is Required?

The basic question, is why transmission is required in an automobile? The power generated by the engine flows through the transmission before it reaches the drive wheels.The basic function of the transmission is to control the speed and torque available to the drive wheels for different driving conditions.

Fig.1 Power flow in an automobile; the power from engine to drive wheels is transferred through a drive train

For example, if you want to climb a hill, you need more torque. By reducing the speed at the transmission, we will be able to achieve higher torque for the same power input. This is simply conservation of energy. Power transmission through a shaft is torque times angular velocity of the shaft. When you reduce the speed of the shaft, it will automatically result in increase in the torque transmission. Conversely, if the torque demand is low , we can increase the transmission speed. These 2 cases are depicted in Fig.2.

Fig.2 During a climb the wheels need more torque; during descent the reverse is the case

The Basic Working Principle

Now let’s look at its inner workings. Manual transmissions work on the simple principle of gear ratio. As shown in Fig.3 a different output speed can be achieved by meshing gears of different size. The speed ratio is given by the simple equation shown in the figure (N represents speed, T represents number of teeth).

Fig.3 The basic principle of a gear pair

Sliding Mesh Transmission

Sliding mesh is the one of the earlier type of manual transmission technology, and the one which is easiest to understand. The most basic slidngmesh transmission mechanism is shown in the Fig.4. Here the input and output shafts are connected through a counter shaft.

Fig.4 First and second gear in a sliding mesh transmission; the red line represents the power flow

This mechanism can operate under 2 different configurations. In the firs configuration, the output shaft will turn at a slower speed than the input. Just by sliding output gear and connecting the output shaft with the input will result in the second configuration. It is clear that, here the input and out will turn at the same speed. Direction of the power flow is represented as red dotted lines in the Fig.4.

A 3-speed mechanism will look as shown in the Fig.5. For the gear meshing shown in the figure, the output shaft will rotate at its slowest speed (1st Gear).It is clear that just by sliding the gears we can achieve different transmission ratios, such as 2nd and 3rd gears.

Fig.5 Three speed sliding mesh transmission: first gear is shown in the figure

The sliding mesh transmission is good for controlling the speed, but they have an inherent disadvantage. It’s quite tricky to slide from one gear and engage with another gear. A technology known as double clutching should be used for achieving a smooth slide of gears. The driver should possess a good skill to do an effective double clutching. Maintenance associated with the double clutch transmissions are quite frequent too.

Solving the Sliding Problem – Synchromesh Transmission

The synchro mesh transmission permanently solves this problem. Here the gears are always in mesh, but with a major difference. Here the output gears are loosely connected to the shaft. You can see from Fig.6 that there is a small clearance between the output gears and shaft.

Fig.6 Synchromesh transmission: Here the gear pairs are always in mesh

If we connect only one gear to the shaft at a time, the shaft will have the speed of the connected gear.

Understanding the basis using a Hypothetical connector

We will first use a hypothetical connector to illustrate how different gear ratios work in the sycnhromesh transmission. Later on we will move to the actual technology. With the help of the hypothetical connector, different gear ratios are illustrated in Fig.5. It is interesting to note that in 4th gear the input and output shafts are directly connected. This means the output and input shaft will have the same speed in 4th gear.

Fig.7 First and Fourth gear are illustrated in this figure with help of a hypothetical connector

The art of locking a loosely held gear to the shaft effectively and smoothly lies at the heart of the manual transmission. Let’s see how this is done in actual practice.

Synchronizer Cone-Teeth Arrangement

First of all, the main shaft gears have a synchronizer cone-teeth arrangement as illustrated in Fig.8.

Fig.8 Synchronizer cone teeth arrangement of synchromesh transmisson

A hub is fixed to the shaft. A sleeve that is free to slide over the hub is also used in this system.

Fig.9 When the sleeve and synchronizer teeth are engaged the locking action can be achieved

It is clear that, if the sleeve gets connected with the teeth of the synchronizer cone, the gear and shaft will turn together, or the desired locking action will be achieved. But during the gearbox operation, the shaft and gear will be rotating at different speeds. So such a locking action is not an easy task.

Use of Synchronizer ring

A synchronizer ring helps to match the speed of the gear with that of the shaft. The synchronizer ring is capable of rotating along with the hub, but is free to slide axially. Before moving the sleeve, the clutch pedal is pressed. This way power flow to the gear is discontinued.

Fig.10 A synchronizer cone is placed between a hub and synchronzier cone

When we move the sleeve, the sleeve will press the synchronizer ring against the cone. Due to the high frictional force between the synchronizer ring and cone, the speed of the gear will become the same as the shaft. At this time, the sleeve can be slid in further, and it will get locked with the gear. Thus, the gear gets locked with the shaft in an efficient and smooth way.

Fig.11 Movement of sleeve brings the synchronizer teeth and sleeve to the speed, after that the locking is achieved

Different Gear Ratios

What we have seen in last section was the technology behind the 2nd gear. In the same way the other gear ratios are also achieved. The details are described in this session.

Under Drive – 1st, 2nd and 3rd

In under drive the output shaft turns at a lower speed than the input. For the manual transmission technology we are explaining 1st , 2nd and 3rd gear ratios fall under the under drive category. The following figure depicts the sleeve motion required for 1st and 3rd gear.

Fig.12 The first and third gear of a manual transmission

Direct Drive

As the name suggests in direct drive, the output and input shafts turn at the speed. For this purpose the output and input shafts are directly coupled using the synchronizer cone-sleeve mechanism. The hub is fixed to the output shaft, when the sleeve gets connected with the synchronizer teeth of the input shaft, they get coupled together. During the direct drive, the sleeve at the third gear position (2nd part Fig.12) should move to left side.

Over Drive

A 5th gear is used to turn the output shaft at a higher speed than the input shaft. You can note here that unlike the other gear pairs, in 5th gear the output shaft gear is smaller than the counter shaft gear. This generates the overdrive scenario.

Fig.13 The arrangement of 5th gear

You can note more difference in the 5th drive configuration, the output gear is fixed to the shaft and the counter shaft gear is loosely connected. As a result synchroizer ring – sleeve mechanism is arranged on the counter shaft. The sole purpose of such an arrangement is to accommodate the reverse gear mechanism. We will see that in next session.

The sleeve motion is controlled by a shift stick . You can also see the mechanism used for controlling the sleeve with the shift stick. You can note that using this mechanism, not more than one sleeve will be engaged with the output gears. That is important, since engaging 2 sleeves at a time will lead to an impossible turning condition.

The Reverse Gear

Now let’s see how the reverse gear works? The reverse gear uses a 3-gear arrangement, as shown. Out of those, one is the idle gear.

Fig.14 The three gear arrangement of a reverse gear

It is clear that addition of one more gear will turn the output shaft gear in the reverse direction. For engaging the reverse gear the idle gear is pushed and connected to the other 2 gears. Thus the required output shaft rotation in the reverse direction can be achieved. Please note here that the reverse gear does not have a synchronizer ring mechanism. This means that, the gearbox rotation has to stop completely before applying the reverse gear.

Fig.15 The idle gear is pushed and connected with the other 2 gears to achieve the reverse operation

You might have noticed that in reverse gear, your vehicle moves in a very low speed. As you can see from the figure the three gear arrangement gives speed reduction in 2 stages. This results in very low output speed (high torque). Generally the reverse has a gear ratio of 4:1 (input speed : output speed).

Diesel Engine vs Petrol Engine

Petrol and diesel engines are the two most commonly used internal combustion engines. Even though their operation seems similar, they have some interesting differences, and each has advantages over the other. This video is aimed at exploring these differences and advantages from a scientific point of view. By the end of this session the reader will be able to predict what happens, if he puts petrol in diesel engine or vice versa.

A detailed webpage version of the video is given below.

The Basic Operation - 4 Stroke Engine

Before going to the main topic let's have a look at the basic operation of a 4 stroke I.C engine. Both engines have the same basic 4 strokes: intake, compression, power, and exhaust. During the intake stroke fresh air is sucked in (or forced in) to the cylinder. The compression strokes compresses this gas and produces a hot gas. Fuel is burnt in this hot gas and the power stroke happens next. Please remember power stroke is the only stroke where the piston absorbs energy from the fuel. The last stroke is to eject the burn gas to the atmosphere. All these operations are depicted in Fig.1 in detail.

Fig.1 Both the petrol and diesel engines have the common 4 strokes

Why There Exists Two Different Engines ?

There are differences between the two engines due to the difference between the way fuels burn. Petrol is a volatile fuel, is readily evaporates, so it gets mixed with the air efficiently. As a result, just a spark is sufficient to produce smooth combustion in a well pre-mixed petrol engine. As you can note from Fig.2 petrol has a very low flash point. Flash point is the minimum temperature required for a liquid fuel to form a spontaneously combustible mixture.

Fig.2 The difference in chemical property causes all the differences

On the other hand, diesel being a less volatile fuel does not properly mix with air. You can note from Fig.2 that diesel has such a high flash point value. However, if atomized diesel is sprayed into high-temperature air, spontaneous combustion will occur.

Difference between Petrol and Diesel Engine

This means that in petrol engines, the fuel and air should be pre-mixed, while in diesel engines, mixing happens only during the combustion. Due to this reason diesel engines use a fuel injector while petrol engines use a spark plug.

Fig.3 In diesel engines fuel and air comes into contact only during the combustion; In Petrol it is already mixed

Many people have a misconception that in the modern petrol engine technology, gasoline direct injection (GDI), the combustion happens during the fuel injection process. This is wrong; even for a direct injection engine a spark plug is needed. The direct injection technology is just an another way of producing a fine petrol-air mixture. Instead using a carburetor to mix air and fuel these technologies use a fuel injector. The biggest advantage of direct injection method is that the fuel to be sprayed can be controlled very accurately. This will result in great fuel savings.

Fig.4 A close view of fuel injector and spark plug

Why Diesel Engines are Heavier ?

You might have noticed that petrol engines are less noisy and vibrate less compared to diesel engines. This is because the combustion process in a pre-mixed mixture is smooth and propagates well (first part Fig.5). But in a diesel engine, the combustion could begin anywhere in the combustion chamber, and it turns out to be an uncontrolled process.

Fig.5 Combustion is smooth and well propagating in petrol engine, but in diesel it is highly unpredictable

For this reason, to reduce the excessive vibration and noise problem, diesel engines require a more rugged structural design than petrol engines. To normalize the heavy unbalanced power production of diesel engines a heavy fly wheel is often required. This is why petrol engines are always preferred for light-weight applications, such as in 2-wheeler or portable devices.

Why Diesel Engines are More Fuel Economical ?

Since the diesel engine is compressing only the air, it can achieve a good compression ratio without risk of self-ignition. But, in a pre-mixed petrol engine, such a high compression ratio is not possible. As we increase compression ratio of petrol engine the mixture becomes more prone to self ignition. This is known as knocking. Over the period of operation knocking badly damages the engine.

Fig.6 Variation of mechanical efficiency of the engine with compression ratio

The reason why your diesel car gives more mileage than a petrol car is due to the difference in the compression ratio. The higher the compression ratio, the better is the efficiency of the cycle. A qualitative efficiency variation is shown in the graph of Fig.6. This is the reason why diesel engines have better fuel economy as compared to petrol engines.

Petrol in Diesel Engine or Vice Versa ?

An interesting question many people wonder is: What if I put petrol into a diesel engine or vice versa?. From what we have learned so far, we will get a logical and practical answer for this intriguing question in this session

Diesel in Petrol Engine

Diesel in a petrol engine will not even cause firing. The reason is simple. Diesel is less volatile and will not mix with the air properly. In fact you will find it is impossible to make a good diesel-air mixture using carburetor or direct injection technology. This means if you apply spark to such a poor quality mixture, it will not initiate any combustion.

Petrol in Diesel Engine

On the other hand, if you put petrol in a diesel engine, you are spraying a highly volatile fuel into a chamber of highly compressed and hot air. This will lead to detonations rather than smooth combustion. Eventually, the engine components will get damaged. Moreover diesel generally acts a good lubricant for the fuel pump and the injection system. When you put petrol (which does not have any lubrication property) into a diesel car your are actually making the intricate components to wear down over the time. So that’s a big no for petrol in a diesel engine.