EFI Tuning Fundamentals: Volumetric Efficiency
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Volumetric Efficiency
10.21
Please Note: The working presented in these calculations has been rounded to two decimal places. At each point in the working the answer is expressed to two decimal places however the full value has been used to calculate the next answer. If you are following through these calculations and your answers are slightly different to those presented it will be due to these rounding errors.
| 00:00 | - If we take away all of the technology that's been applied to engines over the decades we're left with one simple principle, the amount of power an engine produces depends on how much air we can squeeze into it. |
| 00:12 | Nothing else really matters and everything we do to an engine when we're trying to improve its performance really comes down to trying to get more air into the cylinders. |
| 00:22 | More air allows us to add more fuel which results in a larger combustion event and more cylinder pressure to act on the top of the piston. |
| 00:30 | This is the perfect time to clear up a couple of misconceptions There's a belief in the industry that the amount of power an engine can make is purely dependent on the talent of the tuner. |
| 00:41 | Now obviously whoever is in charge of the laptop needs to understand what they're doing. |
| 00:46 | However the amount of power an engine can potentially make is really an aspect of its mechanical design. |
| 00:52 | Let me explain that a little more thoroughly. |
| 00:54 | An engine is really nothing more than an air pump and the amount of power it can produce is an aspect of how much air it gets into its cylinders. |
| 01:03 | Forgetting for a moment about aspects like variable cam control, there's very little a tuner can do from the laptop keyboard to influence how much air is entering the cylinders. |
| 01:13 | This is part of the engine's mechanical design and we can't change it. |
| 01:17 | Our job is simply to supply the correct amount of fuel to mix with this air and then ignite it at the correct time. |
| 01:24 | With this in mind, if we have two tuners who understand what they're doing, then tasked with tuning the same engine, they should be able to produce almost identical results and this will be the amount of power the engine was designed to produce. |
| 01:37 | There is no magic here. |
| 01:39 | By now you should hopefully understand that the key to engine performance is all down to the amount of air we can get into the cylinders and since getting air into the engine is so important it would be handy if we had a way to measure or quantify how well a particular engine's doing. |
| 01:55 | Luckily we do and it's called volumetric efficiency, often referred to as VE for short. |
| 02:01 | Volumetric efficiency tells us how completely our cylinder is filled during the intake stroke. |
| 02:07 | The basis of this concept is incredibly simple. |
| 02:11 | Let's say we have a 2L engine which means that its cylinders displace a volume of 2L. |
| 02:16 | Now if during a full engine cycle it consumes 2L of air then it would have completely filled its cylinders and we would say it's operating at 100% volumetric efficiency. |
| 02:28 | Let's discuss this in a little more detail. |
| 02:32 | Firstly, you might be wondering why the cylinder would not be completely filled during the intake stroke. |
| 02:37 | There are actually a lot of reasons for this including camshaft design, intake system restrictions, exhaust system efficiency and the need to produce an engine with a wide and usable power band. |
| 02:49 | You have to remember that when a manufacturer builds a car they're trying to produce an engine with low emissions and good economy as well as one that's smooth and easy to drive. |
| 02:59 | Maximum power isn't usually near the top of the list. |
| 03:03 | These considerations all rob the engine of that much needed air and prevent the cylinder from being completely filled. |
| 03:11 | Volumetric efficiency expresses the volume of air flowing through the engine as a percentage of the theoretically perfect airflow. |
| 03:18 | Now if that sounds confusing, let's explain it using an example. |
| 03:22 | But before we do that we need to talk about the measurement units though. |
| 03:26 | Throughout this course the examples I'm going to use will all be done using Imperial Units. |
| 03:32 | Now if you're more familiar with metric units, I don't want you to worry, as it's the actual concept we're discussing that's more important than the specific results we're calculating. |
| 03:41 | So please keep this in mind. |
| 03:43 | Now we can get back to our example and here we're going to use an engine with eight cylinders. |
| 03:48 | We'll need to calculate the engine volume, so let's assume for simplicity that it has a bore and stroke of 4". |
| 03:56 | To calculate airflow we also need to know what speed the engine's operating at so we're going to choose 6000RPM. |
| 04:03 | First of all we need to find out the capacity of our engine. |
| 04:08 | To do this, we need to first work out the volume of a single cylinder which can be calculated by working out the area of the bore and then multiplying this by the stroke. |
| 04:18 | To calculate the area of the bore, we use the calculation Pi, multiplied by the bore radius squared. |
| 04:25 | Now remember that our bore diameter is 4" so to find the radius we need to half this which of course is 2". |
| 04:33 | If we put the numbers into the equation we find that the area of the bore is equal to Pi or 3.142 multiplied by 2 squared which equals 12.57 square inches. |
| 04:45 | Now we can simply multiply this area by the stroke to work out the volume of a single cylinder. |
| 04:51 | In this case the volume equals the bore area multiplied by stroke which is 12.57 multiplied by four. |
| 04:59 | This gives us the result 50.27 cubic inches. |
| 05:03 | So at this point we've calculated the volume of a single cylinder and to calculate the volume of the entire engine we just multiply this by the number of cylinders. |
| 05:13 | This would be 50.27 multiplied by eight which gives us a result of 402.16 cubic inches. |
| 05:21 | For simplicity I'm going to round this to 402 cubic inches or ci for short. |
| 05:27 | Now that we know the engine capacity we can work out how much air will flow through the engine. |
| 05:32 | This is actually pretty easy since the engine should displace 402ci of air for each complete engine cycle. |
| 05:40 | The amount of air moving through the engine will depend though on how fast the engine's operating and for our example we wanted to calculate airlfow at 6000RPM. |
| 05:51 | Engine speed is usually expressed in revolutions per minute so we'll keep the units of time in minutes. |
| 05:58 | This means we're going to measure airflow in cubic feet per minute. |
| 06:02 | The reason we're going to use units of cubic feet per minute or cfm for short is to keep the numbers manageable. |
| 06:09 | A cubic inch is quite a small volume so if we were going to use cubic inches per minute we'd end up with really big numbers that are hard to manage. |
| 06:17 | To convert from cubic inches to cubic feet all we need to do is divide by 1728 because there are 1728ci in one cubic foot. |
| 06:30 | So here's the formula to calculate airflow. |
| 06:33 | Looking at each part of the equation we have airflow which is the airflow through the engine, this is the part we're trying to calculate. |
| 06:41 | We have the engine volume or displacement which we've just worked out. |
| 06:45 | We have the engine RPM that we want to calculate airflow at and finally we have the value 1728 which will change the units from cubic inches to cubic feet. |
| 06:56 | You can see this formula that we've taken the engine RPM and we're dividing it by 2. |
| 07:01 | This is because you'll remember we only have one intake stroke for every 2 engine revolutions. |
| 07:07 | So now we can put the numbers from our example into this equation. |
| 07:12 | In solving the equation we get a result of 698cfm. |
| 07:17 | So now we know that theoretically an engine displacing 402ci capacity should flow 698cfm at 6000RPM. |
| 07:28 | If we now measure the actual airflow on a dyno using an accurate airflow turbine, we may find the result is quite a bit different. |
| 07:35 | Let's say that when we measure the airflow, all of the restrictions we talked about mean that the engine is actually only flowing 572cfm. |
| 07:45 | Now we've measured the actual airflow, we can use another equation to calculate the volumetric efficiency. |
| 07:50 | In this formula, actual airflow is the amount of air the engine is actually consuming while the theoretical airflow is the airflow we calculated earlier where the engine is completely filling all its cylinders. |
| 08:03 | So now we can put our numbers into this equation and if we divide 572 by 698 we get a volumetric efficiency of 0.82 which is 82%. |
| 08:14 | This means that our engine is actually only filling 82% its cylinders at 6000RPM. |
| 08:20 | To put volumetric efficiency into context let's have a look at what sort of numbers we could expect to see from some real engines. |
| 08:27 | VE is affected by a wide range of factors but for most modern four valve engines a VE of 90-100% is about is good as you can expect to see. |
| 08:38 | Two valve engines don't flow quite so well and are generally worse of with VE in the range of perhaps 85-95%. |
| 08:46 | Highly developed race engines with well designed intakes and exhausts may achieve VE in the range of 105-110% or even potentially better. |
| 08:56 | As with a lot of the theory behind engine operation VE has been used as a convenient way of explaining the cylinder filling ability of different engines. |
| 09:05 | When we examine it a little more deeply though, there's a little more to consider. |
| 09:10 | For example, if we look at a specialised race engine or perhaps one using forced induction it's possible to achieve a VE of more than 100%. |
| 09:20 | Now if a cylinder has a volume of 50ci then that's a fixed volume and we aren't going to be able to change that. |
| 09:27 | A VE of 110% is effectively stating that we've fitted more than 50ci of air into a cylinder and this may sound confusing. |
| 09:37 | Since the cylinder volume hasn't changed we obviously can't fit 55ci of volume into it. |
| 09:43 | However since air is a compressible gas, we can fit more air into a given space by compressing it which has the effect of increasing the density of the air. |
| 09:54 | This is what happens in situations where we're seeing VE numbers exceeding 100%. |
| 10:00 | To sum up this module, I want you to understand the concept of volumetric efficiency and how it describes how completely an engine is able to fill its cylinders. |
| 10:09 | In this module you've learned how to calculate the theoretical airflow through any engine and you've learned how the volumetric efficiency can then be calculated. |
