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1、THE FUEL& ENGINE BIBLESUCK, SQUEEZE, BANG, BLOW Not a sexual maneuver, but rather the common description for how an internal combustion engine works. The basic way all internal combustion engines work is to take a mixture of fuel and air, compress it, ignite it either with a spark plug or by self-ig
2、ntion (in the case of a diesel engine), allow the explosion of combusting gasses to force the piston back down and then expel the exhaust gas. The vertical movement of the piston is converted into rotary motion in the crank via connecting rods. The crank then goes out to the gearbox via a flywheel a
3、nd clutch, and the gearbox sends the rotary motion to the wheels, driving the vehicle forwards.The diagram to the left is for reference for the technical jargon that will pop out on the rest of this page. It shows an inline-4 engine with dual overhead cams.NIKOLAUS OTTOIf you want to be pedantic, th
4、e suck-squeeze-bang-blow cycle of a 4 stroke engine should be called the Otto Cycle, after its inventor Nikolaus Otto. The development of the internal combustion engine is quite interesting, and rather than add even more clutter to this page, enquiring minds can read about the history of the interna
5、l combustion engine here. The rest of us will carry on.ENGINE LAYOUTS*曲Here arc some illustrations of the most commontypes of cylinder layout you 11 find in engines today. Singles are typically used in motorbikes, snowblowers, chainsaws etc. V-twins are also found in motorbikes. The triple is almost
6、 unique to Triumph motorbikes where they call it the Speed Triple, or the 675. Inline-fours are the mainstay of car engines, as we 11 as being found in some motorbikes away from the TDC mark depending on how fast the engine is spinning. Note that in some engines, the two marks are simply painted or
7、stamped, and there are no degree markings. In this case, the marks align when the first piston is exactly at TDC.CHECK THE TIMING MARKS FIRSTAfter all that, its worth pointing out that crank timing marks can be way off so its worth confirming that your TDC marker is actually TDC before pratting abou
8、t with the timing. Its not as bad now as it used to be, but in the bad old days, Rover V8s were particularly bad for this, with some being as much as 12 off! So how you do confirm your TDC really is TDC? Small cameras, a good set of feeler gauges, some cash and someone who knows what theyre doing.TI
9、MING MARKS ON CAM BELT PULLEYSThe same timing marks exist stamped into the metal near, and on the pulley on the end the cam. Essentially these marks are used to line up the cam to the correct position when youre changing the timing belt. You have to make sure the engine is rotated to TDC and that th
10、e cams are properly aligned too. If you dont, the cams will spin permanently outofsynch with the engine crank and the engine will run badly, if at all.SPARK PLUGSAnd engine without a spark plug is useless, unless its a diesel engine in which case it uses a glowplug instead. But we re talking about r
11、egular petrol engines here so the next topic to get to grips with is the spark plug. It does exactly what it says on the tin - its a plug that generates a spark. Duh. So why spend time talking about it? Well with apologies to George Orwell not all spark plugs are created equal. Some are more equal t
12、han others. They 11 all do the job but the more you pay, the better the plug. All spark plugs share the same basic design and construction though.The high voltage from your vehicleshigh-tension electrical system is fed into the terminal at the top of the spark plug. It travels down through the core
13、of the plug (normally via some noise-suppression components to prevent electrical noise) and arrives at the centre electrode at the bottom where it jumps to the ground electrode creating a spark. The crush washer is designed to be crushed by tightening the spark plug down when its screwed into the c
14、ylinder head, and as such, it helps keep the screw threads under tension to stop the spark plug from shaking loose or backing out. The insulator basically keeps the high-tension charge away from the cylinder head so that the spark plug doesnt ground before it gets a chance to generate the spark.The
15、type of plug Ive illustrated here is known as a projected nose type plug, because the tip extends below the bottom of the spark plug itself. The other main type of spark plug has the centre electrode recessed into the plug itself and merely grounds to the collar at the bottom. The advantage of the p
16、rojected nose type is that the spark is better exposed to the fuel-air mixture.Ground electrode (ground strap) types.There are plenty of different types of grounding electrodes kicking around in spark plug designs nowadays, from Y shaped electrodes (like SplitFire plugs) to grooved electrodes like y
17、ou11 find on Champion plugs all the way up to triple-electrode plugs like the high-end Bosch items. Theyre all designed to try to get a better spark, and to that end, you，11 now find all sorts of exotic materials turning up too. Titanium plugs, for example, have better electrical conductivity than b
18、rass and steel plugs, and the theory is that they 11 generate a stronger, more reliable spark. Gapping a spark plug. Gapping a spark plug is the process of ensuring the gap between the two electrodes is correct for the type of engine the plug is going to be used in. Too large a gap and the spark wil
19、l be weak. Too small and the spark might jump across the gap too early. Generally speaking, the factory-set spark plug gap is just fine, but if you re running an older engine, or a highly tuned engine, then you need to pay attention to the gap. Feeler gauges are used to measure the gap, and a gappin
20、g tool is used to bend the outer electrode so that the gap is correct.Heat ranges. Something that is often overlooked in spark plugs is their heat rating or heat range. The term heat range refers to the relative temperature of the tip of the spark plug when its working. The hot and cold classificati
21、ons often cause confusion because a hot spark plug is normally used in a cold (low horsepower) engine and vice versa. The term actually refers to the thermal characteristics of the plug itself, specifically its ability to dissipate heat into the cooling system. A cold plug can get rid of heat very q
22、uickly and should be used in engines that run hot and lean. A hot plug takes longer to cool down and should be used in lower compression engines where heat needs to be retained to prevent combustion byproduct buildup.Like the site? The page you，re reading is free, but if you like what yousee and fee
23、l youve learned something, a small donation to help pay down my car loan would be appreciated. Thank you.HOW DOES THE FUEL-AIR MIX HAPPEN? MAGIC?You keep seeing me talk about fuel-air mix or fuel-air charge on this page, but I thought it wise to explain how this happens because it is pretty fundamen
24、tal to the operation of internal combustion engines.The fuel and air are mixed in one of two main ways. The old-school method is to use a carburetor, whilst the new-tech approach is to use fuel injectors. The basic purpose is the same though, and that is to mix the fuel and air together in proportio
25、ns that keep the engine running. Too little fuel and the engine runs lean， which makes it run hot. Too much fuel and it runs rich which conversely makes the engine run cooler. Running rich can also result in fouled up spark plugs, flooded engines and stalling, not to mention wasting fuel. Finding th
26、e right balance normally involves about 10 milligrams of petrol for each combustion stroke.CARBURETORSAdvantages : analogue and very predictable fuelling behaviour, simple and inexpensive to build and maintain.Disadvantages : carburetor icing in the venturi, imprecise fuel metering, float chambers d
27、ont work well if they，re not the right way up. HOW THEY WORK.A carburetor is basically a shaped tube. Theshape of the tube is designed to swirl the incoming air and generate a vacuum in a section called the venturi pipe (or just the venturi). In the side of the venturi is a fuel jet which is basical
28、ly a tiny hole connected to the float chamber via a pipe. Its normally made of brass and has a miniscule hole in the end of it which determines the flow of fuel through it. In more complex carburetors, this is an adjustable needle valve where a screw on the outside of the carburetor can screw a need
29、le in and out of the valve to give some tuning control over the fuel flow. The fuel is pulled through the jet by the vacuum created in the venturi. At the bottom of the tube is a throttle plate or throttle butterfly which is basically a flat circular plate that pivots along its centreline. It is con
30、nected mechanically to the accelerator pedal or twist-grip throttle via the throttle cable. The more you push on the accelerator or twist open the throttle, the more the throttle butterfly opens. This allows more air in which creates more vacuum, which draws more fuel through the fuel jet and gives
31、a larger fuel-air charge to the cylinder, resulting in acceleration. When the throttle is closed, the throttle butterfly in the carburetor is also closed. This means the engine is trying to suck fuel-air mix and generating a vacuum behind the butterfly valve so the regular fuel jet won t work. To al
32、low the engine to idle without shutting off completely, a second fuel jet known as the idle valve is screwed into the venturi downwind of the throttle butterfly. This allows just enough fuel to get into the cylinders to keep the engine ticking over.FLOAT AND DIAPHRAGM CHAMBERS.To make sure a carbure
33、tor has a good, constant supply of fuel to be sucked through the fuel jets, it has a float chamber or float bowl. This is a reservoir of petrol that is constantly topped up from the fuel tank. Petrol goes through an inline filter and a strainer to make sure its clean of contaminants and is then depo
34、sited into the float chamber. A sealed plastic box is pivotted at one end and floats on top of the fuel. Believe it or not, this is called the float. A simple lever connects to the float and controls a valve on the fuel intake line. As the fuel drops in the float chamber, the float drops with it whi
35、ch opens the valve and allows more fuel in. As the level goes up, the float goes up and the valve is restricted. This means that the level in the float chamber is kept constant no matter how much fuel the carburetor is demanding through the fuel jets. The quicker the level tries to drop, the more th
36、e intake valve is opened and the more petrol comes in to keep the fuel level up. This is why carburetors dont work too well when theyre tipped over - the float chamber leaks or empties out resulting in a fuel spill - something you dont get with injectors. To combat this, another type of chamber is u
37、sed where carburetors cant be guaranteed to be upright (like in chainsaws). These use diaphragm chambers instead. The principle is more or less the same though. The chamber is full of fuel and has a rubber diaphragm across the top of it with the other side exposed to ambient air pressure. As the fue
38、l level drops in the chamber, the outside air pressure forces the diaphragm down. Because its connected to an intake valve in the same way that the float is in a float chamber, as the diaphragm is sucked inwards, it opens the intake valve and more fuel is let in to replenish the chamber. Diaphragm c
39、hambers are normally spill-proof.CARB ICING.One of the problems with the spinning, compressing, vacuum-generating properties of the venturi is that it cools the air in the process. Whilst this is good for the engine (colder air is denser and burns better in a fuel-air mix), in humid environments, es
40、pecially cool, humid environments, it can result in carburetor icing. When this happens, water vapour in the air freezes as it cools and sticks to the inside of the venturi. This can result in the opening becoming restricted or cut off completely. When carbs ice up, engines stop. In aircraft engines
41、, there is a control in the cockpit called carb heat which either uses electrical heating elements to heat up the venturi to prevent icing, or reroutes hot air from around the exhausts back into the carburetor intakes. In cars, we dont have /zcarb heat but instead there，s normally a heat shield over
42、 the exhaust manifold connected via a pipe to a temperature-controlled valve at the air filter. When its cold, the valve is open and the air filter draws warm air from over the exhaust manifold and feeds it into the carburetor. As the temperature warms up, the valve closes and the carburetor gets co
43、oler air because the risk of icing has reduced. The symptoms of carb icing are pretty easy to diagnose. First, your engine bogs down at high throttle then it loses power and ultimately could stall completely. You 11 stop on the side of the road and wait a couple of minutes, then the engine will star
44、t and run normally. This is because with the engine off, the heat from the engine starts to warm up the carbs and melts the ice so that when you try to start it up again, everything is fine. COMPLEXITY FOR THE SAKE OF IT.As car engines evolved, carburetors had to evolve to cope with the various dema
45、nds. Its not unusual to find five-circuit carburetors which have become so complex that they re a nightmare to design, build and maintain. That flies in the face of one of the carburetors advantages, which used to be that they were simple. Why five circuits? The main circuit is the one which provide
46、s day-to-day running capability. Its augmented by accelerator and load (or enrichment) circuits which can vary the fuelling to accomodate sudden acceleration or the need for more power (like driving uphill). The accelerator circuit also adds a second butterfly valve in most cases which only opens at
47、 70% throttle or more. Then theres the choke circuit designed to provide extra fuel with the throttles closed when the engine is cold, allowing it to start, and finally the idle circuit which does the same thing but when the engine is warm, to keep it going. On top of all of this, with the introduct
48、ion of stricter emissions requirements came catalytic converters, and these expensive boxes of rare metals just don，t work we11 unless the fuel-air ratio is very carefully controlled. And thats something carburetors just couldnt keep up with. Small wonder then that this mechanical tomfoolery gave wa
49、y to fuel injectionFUEL INJECTIONAdvantages : precise and variable fuel metering, better fuel efficiency and better emissions.Disadvantages : Fairly complex engineering that isnt very user-friendly. Binary on/off functionality at low throttles, which is especially noticable on motorbikes where the throttle becomes snatchy5 and it becomes hard to ride smoothly at low speed. HOW IT WORKS.Compared to carburettors, fuel injectorsthemselves are incredibly simple. They a