Bore vs Stroke - What Makes More Power?

hello everyone and welcome in this video we are talking about borer versus stroke and whether it's better to have an engine with cylinders with a large bore or to have an engine with cylinders with a long stroke okay so first a quick lesson on board versus stroke bore is the width of a cylinder and an engine stroke is the distance the piston travels perpendicularly to the cylinder bore bore and Stroke ultimately determine what the displacement of a cylinder is and you can multiply that number by the total number of cylinders to determine an engines displacement so which is better well if your ultimate goal is to create as much horsepower as possible there are reasons why it is advantageous to go with a larger bore relative to the length of your stroke however if your ultimate goal is to create an engine that is as efficient as possible there are reasons to go with a longer stroke relative to the bore width in order to increase efficiency so we're going to get into a couple of reasons for each in this video ok so first let's start off with power and why you might see an engine with a wider bore versus its stroke make more power than an engine using a very narrow bore here but a very long stroke and so for the purposes of this video we're gonna be analyzing three different cylinder styles all of which have the exact same displacement so our displacement remains constant at half a liter and our middle-of-the-road example here is 86 millimeters by 86 millimeters this is a very common engine cylinder style that you will see in many engines 2 litre inline-four cylinders 3 liter v6 engines you will see 86 by 86 millimeter all the time in road car engines now in order to compare this we're going to look at an example 117 millimetres four by forty seven millimeter stroke this is keeping the board a stroke ratio similar to an f1 engine so the pinnacle of you know automotive performance engines out there revving very high typically they're not going to be this large but I wanted to make sure that our displacements were constant so we increase the dimensions a little bit just to kind of show you know a similar displacement and what each of these can Rev to so then we are going to on the flip side have a point five liter engine with the reverse ratio of an f1 engine you would really see engines like this this is not a very typical design here 63 millimeters wide by 158 millimeters stroke but regardless it's going to help illustrate the points for this video so that's why we're using these different ratios here and so one of the critical things about horsepower is how fast can you rev your engine so horsepower is a function of torque multiplied by rpm divided by 5 - 5 - assuming you are using all English units here and so you can see there if torque is held constant which is not super easy to do but if torque is held constant then horsepower is simply a function of rpm if you can Rev your engine higher you can make more power so that's the ultimate goal here and so by decreasing our stroke lengths we are able to increase our rpm limit why well engines Road car engines don't tend to rev much higher than about 25 m/s once they start revving faster than this much faster than this they start to run into issues and so we can calculate what is the average piston speed for various different examples here so for our three examples we can use this equation the average piston speed is equal to 2 multiplied by the stroke length multiplied by the engines RPM divided by 60 now if we know what our maximum average piston speed is we can plug that in there do some dividing and then suddenly we can figure out what is our peak rpm based on our stroke length and so we have the various different stroke lengths here and we can simply plug in the math 750 divided by our stroke length in meters so that's gonna be 0.04 7 so with a forty seven millimeter stroke our maximum rpm for this engine is going to be about 16,000 rpm for the 86 millimeter stroke it's going to be about eight thousand seven hundred rpm and for our 158 millimeter stroke the rev limit is going to be about 4700 rpm and so as you can see the smaller stroke even though it's the same exact displacement is able to Rev much higher and so because it's able to Rev higher it has more power strokes per second and thus it is able to make more power so typically you will see f1 engines capable of revving higher than 16,000 that's because actual stroke is shorter than this and their actual bore is smaller than this and their displacement is a bit smaller also you don't have to use the max rpm so it doesn't mean that just because an engine can Rev to 8700 rpm with an 86 millimeter stroke that all engines out there with an 86 millimeter stroke will Rev that high there are other limitations but realistically you don't tend to see engines getting much above 25 m/s and as a result you can see how using a shorter stroke will allow you to rev higher and thus make more horsepower alright now let's get into our second reason for why a large bore relative to stroke lengths can help improve horsepower and so that has to do with the size of your valves and how much air flow we can actually get through this engine so we're going to start with our base example here of an 80 millimeter bore engine and we're gonna say that it has 230 millimeter intake valves and then it has 225 millimeter exhaust valves and so using these dimensions we're gonna scale this relative to the size of the different cylinders we have so one is it 170 millimeters 186 and one at 63 and so we scale individually each one of these different intake and exhaust valves and you can see that the largest cylinder bore size will have the largest intake valves at 44 millimeters versus 24 for the 63 millimeter bore and larger exhaust valves 37 versus 20 at the smallest and so if you do some math giving each one of these valves the exact same amount of lift five millimeters of lift so they have an opening created as that valve opens into the cylinder allowing for air flow how much area is available for air to flow in and out of the engine total combining the intake and the exhaust valves well you do the math for the largest for 117 millimeters that comes out to about 25 point two centimeters squared looking at the 86 by 86 you get about 18 point six centimeters squared and then looking at the 63 millimeter bore you get about thirteen point seven centimeters squared so for each one of these as you move to the next one you gain about 35% so as you move from here to here you gain about 35% more area that you have for airflow now at low rpm that's not necessarily an advantage and in fact you can have reduced volumetric efficiency meaning less power less torque less efficiency at low rpm when you have these massive intake valves but once he's starting to get that higher rpm those high loads and you want as much air flowing through your engine as possible so you can make as much horsepower well then suddenly you need the valves to be as large as possible with as much area for air flow in and out of that cylinder and so that's why having a large bore will help you out in creating as much power as possible ok so now that we understand power let's talk efficiency and one of the reasons that I've often heard of why long stroke engines are more efficient is that the amount of surface area that they have relative to the volume inside of them is low meaning that there's less overall area to reject heat to during combustion so more of that heat is turned into useful work in pushing that piston down so I thought okay well the math on this is simple enough we've got each of our three examples here they all have the same volume so whichever one has the lowest surface area will be the most efficient design so you calculate the surface area for each of these and you find that this one has a surface area of 386 centimeters squared this one 349 and this one 370 800 so now we see that as you move away from a square design you're actually going to have an increase in surface area relative to how much volume hem so this design is the most efficient right wrong so what's important is that you have to analyze when is combustion actually occurring so first let's have a quick talk on compression ratio compression ratio is the volume before when the piston is at bottom dead center divided by the volume when the piston is at the very top right when combustion is occurring so in this example we have a compression ratio of 11 to 1 I chose this because it's going to make the math easy for this example so we can just divide by 10 so that means the swept area the stroke length here is going to give us a volume of 10 and then above that stroke length we have a volume of 1 so when is combustion actually occurring well it's occurring when that piston is close to top dead Center so what do our combustion chambers actually look like when our piston is close to top dead center well here's our example so we divide each of these by 10 that will give us our stroke length of our combustion chamber and then we have the widths already from over here and so this is what each of these combustion chambers look like they're all at up dead center but as you can see the low bore long stroke design is closest to a square design when combustion is actually occurring so you can measure the surface area in each one of these scenarios and so for this big bore design we've got 231 centimeters squared for this square design we've got 139 centimeters squared and then for our design right here which we have a very long stroke we have 95 centimeters squared so as you can see there's a huge advantage when combustion is actually occurring that's when you want all of that heat to turn into useful work then less surface area is available for that heat to be rejected to in this case there's plenty of surface area the top of the cylinder as well as the piston can absorb a lot of that heat rather than forcing the piston down in the form of pressure and causing it to create useful work so that is why using a long stroke design will be more efficient because it will allow for having less surface area available to reject that heat to finally let's get into burn duration and so the logic here is the quicker you can burn that air & fuel mixture the more efficient of an engine you're going to have and so the reason for this is if you have your piston at the very top and you burn all of that air and fuel immediately then you have all the time with that peak pressure to push down that piston if it's slowly burning then you don't create that peak pressure until way later on we are not able to use as much of that to force the piston down because got less movement to travel and so not all that useful pressure goes to work so you want peak pressure at the very top and as low of a pressure as possible once that piston has reached the bottom and so a simple explanation for why does a short bore long stroke engine have more efficiency here in a shorter burn duration is that that flame has less distance to travel so if your combustion chamber looks like this well and that flame front has to travel all the way to that cylinder wall well it only has to travel about thirty millimeters in the case of this 63 millimeter border zone versus this design over here about 120 millimeters it's got to travel twice that distance the flame has to travel twice that distance so it's going to take longer meaning your piston has moved down further and you're not as efficient now in reality this is a bit of a simplistic explanation of why this happens what actually happens is a bit more complicated but it's very cool and I found a very cool study conducted by Southwest Research Institute where they took an engine 86 millimeter by 86 millimeter bore versus stroke so like I said a very common engine design and they're able to manipulate this engine so they could increase the stroke length and maintain the compression ratio so 86 by 86 86 by 98 stroke 86 by 115 millimeter stroke and so the only things changing by changing the stroke length in this example this test that they did are the stroke length and the displacement now yes it would be ideal to hold the displacement constant but that's very complicated to do because that means your entire valve train has to either shrink or expand so an easier way to assess what happens in changing your stroke versus vallejo is to look at relative differences based on your displacement by changing that stroke length so that's what they did and the results are absolutely fascinating so they went what they were measuring is how long did it take to burn from 10 to 90 percent of the total mass of air and fuel in that cylinder and so the way you measure this is you look at degrees of rotation of the crankshaft so that's time in an engine how much has that crankshaft rotated and so in this example what's very cool is that by moving from an 86 millimeter stroke to a 98 millimetre stroke they saw that the crankshaft rotated two to three degrees less in order for that combustion from 10 to 90 percent to occur going from 86 215 millimeters stroke they saw that duration decreased from about 3 to 5 degrees of crankshaft angle so very cool and they were able to see an overall specific fuel consumption improvement meaning it doesn't matter what the engine size is specific fuel consumption improvement of about three to five percent simply by changing the geometry that's the only thing that's really changed here the other thing is that they did notice in going from 86 to 115 that depending on the spark timing you could actually have a slower burn and you could actually have less efficient combustion occur so there is somewhat of a sweet spot and in this example it happened to be somewhere around about a ninety eight millimeter stroke of course you know we don't know what the exact number is we just have these three numbers to play with you but cool nonetheless they could improve specific fuel consumption by about three to five percent by changing the stroke length so why does this actually happen well the reason why it actually happens is because for a given rpm let's say you're rotating at 3000 rpm if you have a longer stroke as we know that means your piston speed will be faster and with a faster piston speed you have more turbulent flow within that cylinder so that high turbulence within the cylinder means air and fuel mixes really well and combustion occurs very quickly the combustion is forced to move very quickly because of the turbulence within that cylinder so because and that's actually a measurable difference and so they were able to see that that it was able to burn quicker and as a result of it the engine was more efficient now there are of course exceptions to what I'm discussing in this video so just because an engine has a wide bore doesn't mean that it can't be efficient and just because an engine has a long stroke doesn't mean that it can't make a ton of horsepower but if you isolate those variables individually these are the results that you will see and you will tend to see that a longer stroke will result in more efficiency and a wider bore will tend to lead to more horsepower so thank you all so much for watching and if you have any questions or comments of course feel free to leave them below

Channel: Engineering Explained

Views: 1,077,729

Rating: 4.9387789 out of 5

Keywords: bore vs stroke, bore, stroke, engine, horsepower, cars, engineering, engine tuning, make more power, what's better, comparison, engine bore, engine stroke, efficiency, torque, friction, engineering explained

Id: UV3RwBPqznU

Length: 15min 14sec (914 seconds)

Published: Wed Jan 08 2020