There is a serious global crackdown on the level of emissions permissible from motor vehicles. In the Indian context, the upcoming BS VI emissions norms – set to go live on April 1, 2020 – not just require vehicles to be more efficient, but also offer lower levels of CO2 discharge into the atmosphere.
A method of increasing efficiency of internal combustion engines, while keeping a check on their emissions, is by adopting an optimal fuel supply system. The most ideal level of air and fuel mixture into the engine is a key factor in the power performance of the engine as well as for the amount of tailpipe emissions. While these fuel systems are common for all types of vehicles, we will focus on the two-wheeler segment and the strides it has taken with regards to various technologies.
The traditional method of providing fuel to the engine has been through a mechanical system called the carburettor. A carburettor contains jets that push the fuel into the combustion chambers of the engine, with the amount of fuel flow depending completely on the amount of air capable of being sucked into the system. The carburettor is a purely mechanical system that needs to be tuned and cleaned at regular intervals in order to function properly. Dust and other impurities in the fuel that pass through the filtration systems on the carburettor could clog jets or other components of the carburettor and lead to improper fuel flow to the combustion chamber.
Carburation is a favoured and ideal system in two-wheelers, especially those with single cylinders, since it is a simple mechanical system free of any electrical or electronic devices. In addition, the simplicity of the system also makes it a low-cost fuel system technology. However, the main issue with obtaining the best performance using a carburettor is that it cannot monitor the air-to-fuel ratio for each individual cylinder of multi-cylinder engines. Therefore, a single carburettor can only approximate the best fuel-to-air ratio for each cylinder’s performance, unless there are separate carburettors for each cylinder of the engine.
It must be noted that it is increasingly more flexible to tune a carburetted system to either offer higher performance or more fuel economy. This can be achieved by adjusting the amount of air-to-fuel mixture by altering the jet sizes or increasing/decreasing the airflow. While this feature might appear to be a strong positive for the carburettor, this also poses a major drawback for these systems. A sizeable amount of fuel is either not burnt completely, or burns at a higher/ lower rate than advisable owing to the ability to tune. Any of these non-optimal fuel-burning scenarios lead to increased levels of CO2 emissions, making carburettors dirtier than more modern fuel delivery systems.
ELECTRONIC FUEL INJECTION
As in the case of all areas of the automotive industry, the next generation of fuel supply systems are also infused with electronics to feed the engine with the fuel-to-air ratio. Thus, it led to the birth of electronic fuel injection (EFI) systems that make use of sensors and actuators to precisely supply the right amount of fuel into the engine. There are two main types of systems in use at present – Throttle Body Injection (TBI) and Multi-Point Injection (MPI).
TBI is a system that is similar to a carburettor, where one or two injectors are located in a central throttle body that supplies fuel to the engine through the intake manifold. However, instead of using the engine vacuum to pump fuel through metering circuits as a carburettor does, the fuel is sprayed into the manifold through the injectors in the EFI system.
Meanwhile, an MPI system features individual injectors for each cylinder. The injectors are mounted at each of the intake ports so that the fuel can be sprayed directly into the ports. The MPI system is equipped with a single throttle body that meters the amount of air entering the intake manifold, and matches it with the amount of fuel needed to be delivered as per engine requirements.
The fuel regulation in both types of EFI systems is controlled by a combination of fuel pressure and injector timing. A volume of fuel delivered to the engine depends on the amount of time the injectors are on. Likewise, the fuel delivery is also increased when there is a higher pressure differential between intake vacuum and fuel line pressure, which is controlled by a fuel pressure regulator. The major components that form an EFI system include idle air control valve, throttle position sensor, airflow sensor, cold start valve, warm-up regulator, fuel pressure regulator and fuel injector.
In addition to the simple EFI systems, there have been developments in the two-wheeler segment for components that take fuel delivery systems to offer even more improved riding experience that continues to be clean. Engine control units, ride-by-wire systems and port fuel injection are some newer technologies that further enhance the performance of EFI systems. The usage of these new technologies depends on the cost of the vehicle that they are being equipped into, since most of them are of high value at present times.
However, from all the new technologies of EFI systems, engine control units are witnessing growing demand, since they monitor the engine constantly, and provide information to ensure the engine receives the optimal fuel supply at different conditions. As a result of higher adoption of such control units, the costs of these systems are automatically falling, and will continue to do so with increased market use.
The ride-by-wire technology, also known as electronic throttle control, basically controls the amount of fuel that enters the engine through an electronic mechanism rather than a cable-driven one. It communicates with the engine control unit, instead of the traditional cable throttle control, where the rider has direct control over the engine. The ECU compares and calculates a number of parameters, such as engine power required, current engine speed, current vehicle speed, and gear selected, before evaluating the release of the throttle. It is this ride-by-wire technology that enables manufacturers to implement features such as riding modes (Eco, City, Sport) and electronic cruise control, that enable better driving performance and fuel economy through digital control over the engine.
The Indian market is still heavily dependent on carburettor fuel systems for the majority of its two-wheelers, since the cost of this technology is low. In addition, the fact that our current emission regulations have been met with carburation alone has also made this the primary fuel distribution technology. However, a downside is that the expertise required for tuning carburetted fuel systems is high since there aren’t any electronic monitoring devices to view the air-to-fuel mixture. It is more about feeling the vehicle’s fuel flow condition at idle, low and high speeds, and then manually adjusting the air or fuel levels to bring it to what feels like the optimum condition.
Carburettors can be opened up, repaired and fixed back, while this is not the case with EFI systems, mainly due to their electronic nature. The tools and expertise required to work on EFI systems will be specific in nature, which the entire two-wheeler industry will need to progress towards since this will be the future of the industry. It would also mean that servicing bills would increase marginally, as the technician would now be trained specifically to attend to such systems, and will also need to possess certain specific tools.
However, the BS VI emission norms have a more stringent set of requirements for two-wheelers, among other automotive segments, to adhere to in terms of fuel economy and emission control. While this does not mean that all two-wheelers will transition to EFI systems, as already witnessed in the market, a fair number of vehicles will need to make the move in order to adhere to the required specification of the emission norms. As is with every new technology, the cost of EFI systems, which is high when compared to carburettors at present, will surely reduce over time as adoption rates grow further. The reduction in costs of EFI will also strongly depend on the localisation of development and manufacturing of such systems.
(Inputs from Bosch, TVS Motor Company, Continental)
TEXT: Naveen Arul