suck squeeze bang [Music] blow hi I'm Grant and welcome to the second class in the agk series today we're going to be having a look at piston engines piston engines for aircraft are the same type of engine as you would find in a car the only difference is we use them to drive a propeller instead of using them to drive the wheels in a car but how do they work let's find out there are two main types of piston engine which we see in car as well we've got petrol and Diesel we're first going to focus on petrol engines as they are more common and in a couple of classes time we will have a quick look at diesel engines which work basically the same way anyway but have a few little differences it's worth noting as well that I'm not a car fanatic so some of my terms and descriptions might seem a bit basic to anyone who is really into cars and engines in general but that's just not my thing if you need more info on these engines then I'm sure some car YouTube channel will be able to explain things in a lot more depth and detail if you are interested okay so petrol engines work by burning a fuel air mixture within the cylinders to increase its volume and pressure and push the Pistons down which is then converted from linear up and down motion into circular motion through something called a crankshaft which is attached to the bottom of the Pistons and that crankshaft is connected to a propeller through a gearbox air and fuel from now on we'll just call it gas are let into and out of the cylinder via valves and the gases are ignited by a spark plug simple the way this is done is broken down into four steps which we call suck squeeze bang blow in the induction stage the suck stage gases are sucked into the engine via the inlet valve up here the residual momentum in the crankshaft of the previous cycle is what causes the Piston to be moving down in this way in the first place so yes it sounds like we're starting halfway through but it's a continuous cycle so it's hard to pick an exact starting point for this so as the Piston travels down through the cylinder the available volume increases according to the combined gas law this means that the pressure drops the temperature also drops as the gases we are dealing with here are AR adiabatic which basically means they change in temperature and pressure not just pressure so to make the equation work the pressure and temperature both drop a little bit and as the volume in the cylinder increases this drop and pressure causes a premixed combination of fuel and air to flow into the cylinder via the inlet valve the next stage is the squeeze or the compression phase and as the Piston reaches the bottom of the cylinder it is forced back up by again that residual momentum in the crankshaft and the inlet valve is closed as it travels up the volume decreases and according to the combined gas law this means that the pressure and the temperature of the gas must increase then we get the bang phase or the combustion phase so as the Piston reaches the top of the uh cylinder again a spark plug activates igniting the fuel air mixture the combustion actually happens sort of between these two um phases here this adds a lot of temperature into the system it happens instantaneously in a confined area and in that snap shot in time the volume isn't technically changing and in order for us to satisfy the combined gas law this means that the pressure must increase drastically because we've added all that temperature as well this increase in pressure then exerts a force against the piston and seeing as the easiest way thing to move in the system is the Piston it travels back down the cylinder and that is the driving force behind this whole thing as it travels down the volume increases which reduces the pressure and temperature as the Piston travels down to the lowest point in the cylinder once again this means that the pressure and force at the top of the Stroke versus the bottom of the stroke are different it starts off strong and then it weakens as it travels down the length of the cylinder after the combustion stage or the combustion stroke the Piston is at the bottom of the cylinder and it starts to travel back up the way and as the Piston travels up an outlet valve at the top opens up and as the volume of gases is reduced the temperature rises and the pressure Rises to force the exhaust gas out of the cylinder and out the back of the engine into the atmosphere the residual momentum from the combustion stage Powers this motion and then it leads back to the start of the cycle with a new induction stroke the four sto the four stroke cycle takes two full rotations of the crankshaft to complete and the crankshaft is vital in carrying forward the momentum to power the other Strokes that are not this combustion stage that is just the idealized cycle as well in reality it's necessary to slightly alter the timings of the valve openings and spark plug ignition in order to maximize the efficiency the gases don't allow all burn instantaneously for example so a bit of meddling of the timings is required to get the engine working as efficiently as possible the Pistons and cylinder are the heart of an engine the cylinder is a tube essentially that is airtight and made of a sturdy material as it is used to contain heat and pressure increases caused during that combustion phase of the cycle the Piston fits tightly into the cylinder often with a few seals added to maintain an airtight uh seal and it moves up and down along with a connecting rod and the connecting rod is attached down below off the bottom of the page to a crankshaft one of the major factors in how much power an engine can produce is how much air and fuel mixture can be ignited in the cylinder the more gas is ignited the more power is produced this can be done by increasing the size of the cylinder or we could just add more cylinders in a normal four stroke engine the actual driving force is only created 25% of the time it's only during that combustion Cy uh phase of the cycle which means that only one of the Pistons actually drives the crankshaft at any time if we add more cylinders if we add eight cylinders for example we could figure out the timings of the cycle so that we always have at least two Pistons driving the crack shaft and therefore more power would be produced as a result so the size of the cylinders and the number of the cylinders is important this this is why you often hear engines classified this way you might hear something called I don't know like a 3 l 6-cylinder engine for example the liters refers to the total volume of the internal area the amount of gas that can be burnt within the cylinder and the number of cylinders obviously is just the number of those cylinders other ways of classifying an engine is by the arrangement of the cylinders and there's a few ways to lay out the cylinders in line is the most common and simplest usually four cylinders in a row like we've been looking at uh previously you could also have radial engines this is a circular engine where the cylinders form a ring around a central crankshaft these engines normally have an odd number of cylinders and are quite often relatively exposed so that they can take advantage of the air passing over it to help with cooling you can also get v-shaped engines these are most often associated with high performance engines and it is where the cylinders are arranged in a vshape so you got two Banks of cylinders offset from each other like this it allows for more cylinders to be added for the same length of engine and a shorter engine has a short heavy crankshaft so therefore saves weight if you put them all in a line you'd have a longer crankshaft and that would be very heavy so basically a V8 engine is the same length as an line four cylinder engine so a significant weight saving can be made on the crankshaft because the crankshaft has to be made very sturdily but you get twice the amount of cylinders you can also get flat engines this is like a very opened up V engine and it allows for again that short crankshaft but also as the name suggests it's flat so the engine can be contained in a nice streamlined engine cover and reduced drag making these engines very common in light aircraft all Arrangements of the cylinders the radio the V and the flat have their place in the world of Aviation and one will be picked over the other by the manufacturers for various different reasons that work with the design overall of the aircraft spark plugs use an electrical current to create a spark and ignite the air fuel mixture in the cylinder they draw their power from the battery and the batter is charged up while the engine is running via something called an alternator or a Magneto system can be used which is end independent of the battery and we'll have a look at a bit more detail in the next class there are usually two spark plugs per cylinder for redundancy purposes basically if one fails there's another one to still make that cylinder work um and that's one of the most common things that can happen these are uh quite cheap components so they fail fairly easily and the most common way that they fail is through something called fing and this is where oil or fuel or other contaminants make the spark plug very dirty and basically block off the area that is used to create uh that electrical spark if we have two spark plugs as well all also increases the efficiency of the engine as the gases in the cylinder are ignited from both sides simultaneously which creates a more even burn in this area and allows the Piston to uh be propelled down the cylinder in a much more even fashion the crankshaft is connected to the the Pistons via the connecting rod of each piston crankshafts need to be very sturdy and are made of heavy steel Alloys normally in order to withstand the force of the Pistons as they move up and down this is why it's beneficial to use those v-shaped engines or those flat engines as more cylinders can act on the same length of crankshaft therefore saving weight um from that heavy steel uh heavy steel Alloys the crankshaft is basically used to trans translate the up and down motion of the Pistons into a rotating motion that is then connected to a gearbox which will then be connected to a propeller I found the crankshaft quite hard to draw it's not really a straight line it's more like a zigzag sort of shape and there's counter weights uh on the side the opposite sides of where the connecting rods and the Pistons are attached have a little Google search for what crack shaft actually looks like in 3D and you'll see what I'm talking about it's a a squiggly line of metal usually machined or um cast as a single piece of metal to help with its strength and it rotates in a very uh interesting way it's worth having a look at a 3D animation or something like that if you can get your hands on one so a cam shaft or multiple cam shafts are connected to the crankshaft via gears and it runs at half the speed of the crankshaft this is because it is used to time the opening and closing of the inlet and exhaust or Outlet valves the valves are spring-loaded into the closed position and then they are opened up at the correct time via the cam shaft and push rods or the cam shaft can be uh directly connected to the valves and open them when the time is correct basically what you've got is um sort of a round pointed Circle I don't know what you would call that it's almost an egg shape and when the egg uh reaches the top of the cycle like in this it will push the push rod and this it'll go over the rocker arm push the valve down allowing air to flow in and then as it continues its rotation the uh connecting rod will sort of fall down and the spring will push it back down in order to uh close the valve again it's like the crankshaft it's quite hard to draw and explain it's better if you can see a 3D model of how they work you've got these egg shaped things rotating pushing the push rods at the correct time which then open and close these valves or you can have the cam shaft directly mounted if the push rods weren't here you could put these sort of egg shapes directly onto the top of these valves and they would open and close them as they rotate the valves themselves are designed to be airtight and withstand the high temperatures and pressures involved in the combustion stage obviously in summary you've got a piston engine working through a four stage cycle suck squeeze bang and blow in the suck phase the inlet valve opens allowing air and fuel into the cylinder in the next phase The Squeeze phase the Piston travels back up with both the valves closed increasing the temp and pressure of the fuel and air mixture it then ignites instantaneously because the spark plugs set off via some electrical source this increases the temperature a lot in the gas the increase in temperature causes the volume and the pressure to increase so the easiest thing to move in the system is this piston so the pressure is felt as a force by this piston and it moves the Piston down is connected to the crankshaft via a connecting rod and the crankshaft translates the up and down motion of the Pistons into a circular motion which can then be used to drive a gearbox propellers and in cars uh Wheels after the power stroke the bang phase you get the exhaust stroke so the residual motion of the crankshaft forces the Piston back up and the valve opens up the exhaust valve opens up to allow all that uh spent Fuel and air out of the system and out the back of the engine into the environment via the exhaust pipe the valves are controlled via something called the cam shaft these are sort of spinning egg-shaped things that push the valves at the correct time in order to open them and close them when appropriate the idealized cycle uh things happen exactly in this four stage cycle as I've said but in reality the timings have to be adjusted slightly so that um the um practicalities of the real world can be brought into it like for example the gas doesn't all ignite instantaneously so you might have to lead the spark plug slightly so instead of happening at the point of uh when the gas is most compressed you do it just before it's at the most compressed and then the top bit will ignite first and then the rest will ignite as the Piston starts to travel little things like that adjustments to timings all make these engines way more efficient and the cylinders themselves can be laid out in a few ways you've got the inline engine like we're talking here you get the radial engine where they're all arranged around the crankshaft in uh a spiral H in a ring not a spiral and then you got the v-shaped engines which have the advantage of adding more cylinders without adding more length of crankshaft and you get the flat engines which are like a V engine opened out even more and that has the same benefits of the V engine but the engine isn't tall it's quite flat as the name suggests and that means you can get a nice aerodynamic housing around it which means it's very common in engines for propeller driven aircraft