We all love to hear our engines scream with power. The roar is nothing but the sound waves created in a harmonious explosions taking place in the combustion chamber of the engine. When this harmony is disturbed due to a variety of reasons, what results is Misfiring of the engine, just like a skipped heart beat.
I wanted to share with you the knowledge/learning about why this happens and how to avoid this. The word “Misfire” literally means missed an instance of fire or back fired or a failed explosion. Before we actually start to discuss more about the word misfire (in regards to the IC engines), it is important for us to understand how a 4 stroke engine works (single cylinder petrol engine, to keep it simple).
1st stroke: Suction
The piston from TDC (Top Dead Centre) starts moving towards BDC (Bottom Dead Center), the inlet valve opens allowing the air fuel mixture to enter into the combustion chamber until the piston the reaches the BDC and the inlet valve closes.
2nd Stroke: Compression
The piston now along with the air fuel mixture starts moving towards the TDC from BDC compressing the air fuel mixture till it reaches TDC increasing the temperature of the air fuel mixture just right to catch fire when the spark happens. Both Inlet and Exhaust valves are closed ideally.
3rd stroke: Power stroke/Combustion
As soon as the piston reaches the TDC spark occurs which initiates combustion of compressed air and fuel mixture, the expansion of high temperature and high pressure gases produced by this combustion causes the piston to move towards the BDC and the force generated here is utilised to propel the vehicle etc. Inlet and exhaust valve still remain closed.
4th stroke: Exhaust Stroke
Once all the air and fuel mixture is burnt inside the combustion chamber and the piston has reached the BDC, the energy stored in the crank/fly wheel pushes the piston towards the TDC to expel all the burnt gases and allowing the exhaust valve to open, all the gases are expelled through the exhaust valve by the time the piston has reached the TDC and the exhaust valve closes keeping the piston and the combustion chamber for the Stroke one which is Suction Stroke and the cycle continues.
Wow! That was quite a bit, reminds me of my college days. 🙂
It is also very important for us to understand how the air and fuel mixture is sent to the engine’s combustion chamber and the burnt gases are sent out before and after the combustion respectively.
There are two major ways to supply the right amount of the air fuel mixture in to the engine, i.e. either using Carburetors or Fuel Injectors controlled by an ECU (electronic control unit). We all know how the exhaust systems guide the burnt gases out from the combustion chamber, nevertheless we will discuss a little bit on this later.
The intake, like we mentioned above is managed by either of the two components.
There are many types of the carburettors manufactured for a variety of motorcycles. However, the most common and the recent ones are known as Constant Velocity (CV) carburettors. These carburettors work on the Bernoulli’s principle maintaining the speed of the air constant.
There are mainly the two jets involved in controlling the fuel flow in the carburettors namely “Pilot Jet” and the “Main Jet”. For a quick understanding, the pilot jet is the one that is responsible for the fuel flow when the engine is idling and main jet takes over while there is load on the engine/throttling. This is caused due to varied pressure in relation to the ambient/atmospheric pressure at the tip of theses nozzles. The vacuum created by the engine during its working cycle uses the fuel flowing from the pilot jet in to the carburettors Venturi chamber (Bernoulli’s principle) where the passing air mixes with it and is guided inside the combustion chamber through the inlet ports while idling.
Pilot Jets are the smaller jets with smaller inner diameter and allow less fuel to pass through them, unlike the main jets are bigger in diameter and allow more fuel to pass through them while throttling. So when the user twists the throttle the throttle body inside the carburettors opens allowing a sudden change in the pressure usually lower than that of the ambient and that during idling. The deficit change in pressure is much larger than the idling pressure change in comparison with the atmospheric/ambient pressure. This causes the fuel to flow from the main jet and similarly enter the venture chamber of the carburettors to mix with the air and enter combustion chamber of the engine. That’s a very brief explanation of the working of a carburettors.
Imagine a vacuum cleaner when switched on has a low pressure at the tip of its nozzle because the air is being sucked out from a motor on the other end. This is very similar to the engine running in idling (no throttle), however when you point the nozzle of the vacuum cleaner on the floor to clean there is a sudden increase in the sound and the suction force. This is the same principle how a fuel flow shifts from pilot to main jet due to the change in the pressure to pull more fuel out of the carburetor’s fuel collection chamber.
Now moving on to the new aged smart carburetors, that is what I like to call them, the fuel Injectors controlled by ECU. There is a whole new ball game when it comes to these fuel injectors. Fuel injectors as mentioned are controlled by ECU (electronic control unit), they are smart because they take input from a lot of sensors which are strategically placed in and around the engine and calibrate this data as per its program or its mapping and send a desired amount of fuel to flowing air into the engine. Everything here is pre-planned.
ECU’s primary task is to control the air-fuel mixture, control idle speed and Ignition timing and in some case the valve timings. Before we understand how it does that lets go through a flow diagram to see what all an ECU needs to control a fuel injector’s functions.
The ECU as shown above takes inputs from the sensors such as MAP (Manifold Absolute Pressure) sensor for the pressure depression (remember I mentioned about the pressure deficit while explaining about the carburetors), MAF (Mass Air Flow) sensor for the amount of air flowing in, TP (Throttle Position) sensor, Crankshaft Position Sensor and O2 sensor. The inputs from all these sensors help the ECU to determine the fuel required for the amount of air that is flowing in to make it stoichiometric (ideal air fuel mixture), Then accordingly activate the fuel injector by sending appropriate duration of pulse to actuate the solenoid’s electromagnetic field. This attracts the plunger towards it thus allowing the fuel to flow out in to the stream of air and mixes with it and enters the combustion chamber through inlet valves.
That’s majorly the two options to send in the charge (air fuel mixture) into the combustion cylinder. If this charge comprises of too more air and less fuel, then the mixture is said to be Lean and the vice versa of the same is called Rich mixture. The ideal mixture ratio is 15:1 i.e. for every one gram of fuel 15 grams of air is required. If the mixture has air anything above 15 then its lean and anything below is rich.
Although a slight rich mixture is not so efficient but generates more power and is cooler on the engine but the leaner mixture although economical are the ones that heat up the engine and decrease the life by a greater margin and adds to the pollution by releasing higher volumes of nitrogen.
This is where the topic gets interesting, The Phatakas! (intentionally creating a misfire in the engine)
The guys with a kill switch make the engine shut down its supply of spark and fuel which causes a big blasting sound accompanied by the cool looking flame from the exhaust pipes. Just imagine the level of torture their motorcycle engine undergoes, let alone the nuisance that is caused to the public. Their mechanics would be happy making a lot of money on servicing these motorcycles every time.
By cutting the power and fuel, the amount of charge that gets into the cylinder is way too lean and detonates an uncontrolled blast in the cylinder sending a shock of the burst out of the exhaust valve into the exhaust system. The un-burnt oxygen that comes out of the engine through the exhaust valve comes in contact with the exhaust system’s inner wall (if not insulated well), catch fire by increasing its temperature which causes them to exit the system with a lot of backward thrust. These all collectively hinder the working of the valves of the engine. This is the same kind of thrust felt on the shoulder produced by the exiting bullet of gun. Now that is something that can be controlled but constantly abusing the engine with this irregularity takes a big toll on the engine. It first starts affecting the exhaust valves, then subsequent parts of the engine connected to them later resulting in disrupting the surface of the piston and the cylinder walls.
The other misfire that you hear especially when you change to any aftermarket exhaust is mostly because the intake not tuned in relation with the exhaust. The intake (air-fuel mixture) can be easily tuned in a carburetor by adjusting the air screw, it is a trial and error process mostly on the 2nd or the 3rd iteration you may find it comfortable. There is no thumb rule for the number of twists to the air screw because every engine is different depending on the owner’s usage although the engines has same specs. The approach remains the same, however with the motorcycle’s that come with an EFI, remapping is the only way. Meaning reprogram the ECU for the desired need and eliminate the misfire. Although this looks very simple, it takes a skilled personnel to carry this procedure, mostly the company’s showroom should help you with this.
Royal Enfield’s beginning with the BS4 models sold in India with the EFI’s come with the O2 Sensor in the motorcycle exhaust which till date was only available on models sold outside India, “The O2 Sensor (oxygen sensor as shown in the picture below).
This ECU although gets a feedback from the TPS & MAFs, lacks a fine tuning option depending on the output of the engine that is only provided by these sensors. The ECU takes in a fine calibration of settings to fine-tune the fuel flow to make the air fuel mixture appropriate at every descending RPM of the engine and eliminate the misfire from the engine.
Now if you want to fix the misfire even without a O2 sensor then it becomes very debatable depending on the tuner. There are a lot of remedies available in the market for the same, one of them very popular is to go for an add-on feature to the current ECU which gives 5 different map settings from economical to power mode but again it will lack the O2 sensor to keep calibrating the input depending on the output. It is just a 5 different mode mapping. The rich mixture may help to bring down the misfiring however may not be economical (mileage friendly).Finally, it is the common sense and basic knowledge of the rider that decides how he/she understands the health engine and keeps away from the Phataka!!Remember, next time someone shows off their Phataka skills, you know they are commiting a “Homicide of their Engine!!”