Car Modding, Car Tuning and the Science of Horsepower - Electric Superchargers
I’ve been lurking around automotive forums long enough to see this topic come up several times. Someone will inquire about a product they found online, labelled as an “Electric Supercharger” or “Electric Turbocharger” and ask for feedback as to whether the product holds water. At that point, various other forum members will engage. This is done in one of 3 ways. Either they will cite previous scams (of which there are PLENTY), they will say it can’t be done and you’re a fool to try (thereby shaming the original poster into changing his opinion) or they’ll take a bunch of cheap shots at both the OP and the product, starting a flame war. The last one is the most ignorant and the most destructive because it ends up making “electric supercharging”, "joke" and “scam” synonymous through malice and uneducated arrogance.
So what am I trying to prove here? Well, let’s get into a bit of detail first and hopefully the truth will reveal itself without a whole lot of banter from me.
A belt-driven blower at work
What is electric supercharging?
Electric supercharging, in principle, is delivering forced induction or “boost” to your car just like any conventional supercharger or turbocharger would, except that the power to run the unit isn’t coming from exhaust gas back pressure or from a belt attached to the engine. It’s coming from a battery and an electric motor.
Why do this? There seem to be a couple of distinct advantages. First off, you can turn the unit on and off whenever you want instead of being forced to run it all the time. This can be a tremendous fuel saver because the car can run as a naturally aspirated car would, most of the time until you need boost and not before. The second big advantage is that there are no engine losses to speak of when operating the unit. Turbos create back pressure in the exhaust line, which the pistons need to fight against in order to expel more exhaust. This robs the car of some power in order to ultimately produce more. The supercharger is even worse because its less efficient belt-driven drive takes power directly from the engine rather than scavenging thermal energy from the exhaust. Again, the engine is literally giving up power only to gain it back again, and then some. The result is wasted gas, more mechanical strain on the engine and less potential for timing advance due to the extra torque fighting the pistons.
At this point some people might be raising their hands saying “Hey hey, electric supercharging isn’t lossless… you have to run this thing off the car battery!”. Some units require you to do this. Others don’t. For the ones that do, yes you’re leeching some energy from the battery to run the electric supercharger and you really should not do this if you want a completely lossless system. You should use a separate battery. However, the energy you are using from the car’s primary battery is being used to increase air pressure and there is a misconception that the number of Watts or HP you’re pulling from the battery are as much or greater than the Watts or HP that the car gains in performance (conservation of energy). This is wrong. It’s an incorrect application of the notion of conservation of energy. What the car actually gains, comes from GASOLINE, not air. The chemical energy of the EXTRA gas you can inject into your engine, resulting from the extra air you’re adding, is where the power comes from. Therefore you could “spend” 2 HP in electrical battery power driving a supercharger and get 10HP in vehicle gains because the additional air is only a catalyst that allows you to realize the chemical potential energy of gasoline, which took hardly any energy at all for the injectors to squirt into your engine.
This argument is more or less academic though since it really is a good idea to run the system off a separate battery. This way, no engine power is used at all. So let’s recap. Electric supercharging has the potential for being convenient (instant on/off capability), efficient (no losses from the engine) and it is more forgiving for future mods because it puts less strain on the engine for the same level of boost, which increases modding overheard. Furthermore, they are comparable in weight or lighter than supers/turbos and cost the same or less in terms of $/HP. This is all fine and great because we’re talking about the technology itself but how do the products stack up?
The Small Toys
You can find tons of so-called “electric superchargers” on the net. Some brands are more popular than others and the prices range from about $50 to $400 or so. These units seem to come in 2 flavours. The first design is something akin to a turbine that is placed in series with the induction tubing plus a high flow cone filter on the end… except that real turbines rotate extremely fast under incredible power and have multiple stages of compression. These units do not. In fact they are more closely related to a linear fan with more blades, made of cheaper materials, running slower, leaner and with looser engineering tolerances than any true turbine. The other type of product offered resembles a small turbo. It’s a centrifugal design with an inexpensive impeller, driven by electric motor. Now the linear fan guys quote figures like air outlet speed and volumetric flow (you may have seen CFMs being used a lot) while the centrifugal blower guys talk about how many extra PSI you will gain in pressure from their units. In all cases that I’ve come across, they are designed to run off the existing car battery, which, as I said above, isn’t evil IN PRINCIPLE, it’s just not a great idea.
Now as for claims, the most common ones I see are gains of 15%, adding 3-5 PSI of boost or flows up to 1600 CFM of air. I won’t credit these figures to any particular source but they give you an idea of the scale of performance attributed to these products. The first claim is fairly worthless as they could attach their unit to a lawnmower to get 15% if that only amounts to 1HP. The extra pressure claim is actually impossible as you’ll see later, simply due to the current draw of the unit and the fact that such electrical power is insufficient to compress air to those levels above atmospheric. I’m referring to products that draw in the neighbourhood of 50 or 60 amps, which appears to be the status quo for electric superchargers in this price range. Finally the volumetric flow claim of such and such CFMs is useless when speaking of performance. It’s useless because it’s a rate of VOLUMETRIC flow, not MASS flow. Extra air molecules and therefore air mass will give you more power because there’s more stuff there. Speaking of volumetric flow is like talking about the dimensions of a bottle and then not telling you what’s in the bottle. It could be water or air or molten plutonium. There’s a big difference in the mass of each but they reside in the same volume of space. For engine performance you need to speak in terms of mass flow or at least talk about volumetric flow AT a certain pressure and temperature. For example, 400 CFM tells you nothing but 400 CFM @ 14.7 PSI and 27 degrees C does!
As you can see, the manufacturers shoot themselves in the foot by making claims that are either impossible or irrelevant. At this point I should mention that every engine has different air demands so it “may” be possible for the finest products in this price range, drawing up to 125 amps or so (the very highest I’ve seen), to increase volumetric efficiency in some smaller engines by alleviating losses in pressure head across the filter media and from skin friction along the length of the induction tubing and intake runners. In theory anyway, smaller cars could even enjoy a lightly boosted effect but it wouldn’t come anywhere near their claims and would be a miserable failure on bigger engines that require more air. More importantly, any theoretical gains are attenuated largely by compressor construction and efficiency as well as battery losses. At least with a Roots blower, backflow is insignificant so you get the air mass you expect but with linear and centrifugal blowers, air recirculation, vorticity and backpressure can limit boost quite severely, especially at low power, making the free-flowing characteristics of the unit absolutely irrelevant. By free-flowing characteristics I mean filling up a garbage bag with air in less than a second, something the manufacturers use as a marketing gimmick in demonstration videos. That’s easy because there’s no resistance. Try to cram more air into an engine that doesn’t want it and you’ve got yourself a problem. In this way, what you see in real world gains is a mere fraction of the calculated gains.
Case Study - The Leaf Blower
A few attempts have been made to use an ordinary leaf blower as a supercharger, which is an awesome idea, even if it does void your insurance ;) In EVERY case that I’ve found on the net, the unit being used is either gas powered or battery powered. I will tell you right now that any leaf blower running off a battery is much weaker than one running off of a 120V house outlet and certainly weaker than a gas unit. Nobody every pays attention to this detail though and because of that, the battery driven units fail to produce any extra power and promote further criticism of the concept in general. The attempts made with a gas powered leaf blower have been intriguing, almost promising and their portability makes them great as a bolt-on or novelty mod. I believe that the leaf blower is the “entry point” into electric supercharging, if you will, because the more powerful electric and gas powered versions are actually constructed well enough and draw enough raw power to be more than a decoration on the car. I tried to prove it on my own car, for myself and for others. I had my car up on jack stands, my leaf blower plugged into the wall and a custom coupler sealing it to my intake.
The trial however was unsuccessful only because I realized how stupid I was in thinking I could run this test without a dyno. Without resistance on the wheels, they spun wildly out of control with almost no throttle and caused the car to shake. I was afraid the car would fall from the stands so I aborted the test. The next thing I tried to do was run the same leaf blower off a battery using a 1500 W inverter but the stupid inverter didn’t like supplying 120 amps and shut itself off. I gave up at this point although my inability to complete the test does not disprove the idea. It merely leaves it unproven. I was prepared to do a whole bunch of measurements using my OBDII tuner that would have really shed some light on whether leaf blowers could deliver.
In the mean time, since this is a case study, let’s look at the math that I went through prior to this test and see what it tells us:
Assume that we are running a 120V, 12A leaf blower (200 mph rating or 89.4 m/s) from a car battery and sufficiently large power inverter. Unless stated otherwise, all conditions will be assumed ideal. I’ll note any non-ideal conditions that are significant. The engine being considered for this calculation will be the 3.4L GM LA1 that powers the Grand Am, Alero, Aztek and others. The air properties will be 27 degrees C and 14.7 PSI or 101300 Pascals.
This engine draws a maximum of about 160 grams per second of air for all its cylinders. I didn’t make that number up. This is what the MAF sensor is actually measuring as an average across multiple scans. It usually occurs somewhere in the 5000 RPM area, before shifting. By using this MAF value, we can avoid using ideal values or messy conversions from volumetric flow to mass flow because we’re already given mass flow with this reading. The engine needs 160 grams of air each second it operates at maximum power. If your blower can force more than that amount of air into the engine, you will see a gain, period. Again, bear in mind that gains will be more pronounced before max power is achieved because the demands of the engine are lower when the RPM is lower, allowing for greater gains as a percentage of what you’d normally get at that point in the power band. The velocity of air going through the intake will be derived from dm = ro x A x V. Solving for V gives you 29.7 m/s. The kinetic energy of the naturally aspirated flow therefore is E = 1/2mv^2 = 141.4 J (in a 1 second packet of air). I’m modifying a component of Bernoulli’s equation to give a time-based energy instead of a volume based one.
So we know we need at least 160 g/sec of air. Let’s take a look at the electrical calculations now. Power, P = VI, where V is voltage and I is current so P = (120V)(12A) = 1440 Watts. This is the electrical power needed to run the leaf blower. Now a centrifugal blower has a wide range of efficiencies that depend on pressure ratio, flow rate, design, tolerances, etc. I have no idea what the efficiency of a household leaf blower is but for the sake of argument let’s compare it to a turbocharger and say 75%, an optimistic figure but not entirely unrealistic. This number will drop significantly at higher pressure ratios but for low boost, it’s ok. Electric motors and power inverters are very efficient so I’m going to use 90% for each of them. Already we have 0.9 x 0.9 x 0.75 = 0.6075 or about 61% efficiency. This is referring to how efficient the conversion is from battery energy to mechanical energy used for air compression. In other words 61% of your total input is realized as actual boost. The rest is wasted as heat.
Stupid obviously but oddly intriguing...
Leaf Blower Apocalypse!!!
A Real World Electric Turbo Install in Progress
Now 61% of 1440 W is 875 W. Since this number is greater than the 141.4J kinetic energy of the N/A flow (in 1 second), we already know the leaf blower will not be an obstruction. That is, it will not reduce flow and performance by getting in the way. Just to be sure though, 875 = ½ mv^2 = ½ m (89.4)^2, so m = 219 grams. Yep, this is more air in a second than the car was drawing before. We will now convert 875 W of electrical energy into thermal energy because thermal energy is a commodity that can be translated into work (mechanical energy). We will also assume the change in air temperature is negligible across the blower, even though it will change, because we don’t know the air properties at the outlet of the compressor so we cannot do a proper thermodynamic analysis. The best we can do is approximate the change in mass flow rate based on the change in enthalpy of the air across the compressor. The energy in Q1 = m x h1 where m is the air mass and h1, the enthalpy of air at 27 degrees C. From ideal gas tables, this is 300.19 kJ/kg. With an air mass of 160 grams, we have Q1 = 48030 J. If Q2, the energy leaving the compressor is Q1 + 875 J (875W for 1 second) and the compressor is assumed adiabatic then Q2 = 48905 J, which is a 1.8% increase over the inlet air’s energy. Although this does not directly correlate to a boosted mass flow rate or an outlet pressure (we lack data to solve for those values), we can approximate the power gains to be about 1.8% or 3 HP when the engine is already producing maximum power. At 93 grams per second (closer to the beginning of the power band), the same leaf blower would make about 3.1% in gains. Again I must strongly note that all naturally aspirated engines suffer pressure head losses from having to suck the air in on their own. This natural suction will INITIALLY be very easy to overcome by applying some light boost. The gains you may see from a leaf blower or comparable electric unit are mostly offsetting the inherent losses of the system and increasing volumetric efficiency.
So these numbers aren’t very impressive obviously but they are gains (or reduced losses) and remember this is a decently sized V6 engine, not a 4-banger. It wouldn’t be a very economic choice for a gain of 2-3% in HP but it would be funny :)
So what have we learned from this case study? Well, what I consider to be a “lossless” solution for not drawing energy from the engine is still lossy in and of itself so if you don’t have big gains to start with, you lose too much to even notice what’s left. That’s why even the best of the ebay chargers, drawing 125 amps or so (like our leaf blower) just can’t justify its own price tag. Engineering design and manufacturing quality are so important for small power adders too because you really need to squeeze as much as you can out of your product and to do that you have to have tighter tolerances, better materials and ultimately inflate the price even more. It’s just not worth it in the end.
Now this is an electric supercharger!
The Big Boys
All hope is not lost however and there are ambitious projects taken up by dedicated individuals to make a serious attempt at boost electrification. One such undertaking is a product offered by Thomas Knight and is a testament to the amount of resources and power you actually need to make a decent electric supercharger.
The product, prefixed by ESC, comes in different tiers for different sized engines. The ESC-350 is best suited for engines demanding 350 CFM @ +4-5 PSI or less while the ESC-550 is more appropriate for engines that need 550 CFM @ +4-5 PSI or less. There are also ESC-750 and ESC-1000 models. There's even an ESC-150 for motorcycles and other small applications. All models consist of a modified centrifugal compressor, driven by a custom wound electric winch motor. All models MUST run off of auxiliary batteries and in some cases require many batteries to get the correct voltage and current drain capacity needed to power the unit. When enabled, the supercharger runs anywhere from 30 seconds to 3 minutes or more before depleting the batteries. The variability here is because each model has a different battery configuration and each engine will be loaded differently. A quarter mile race usually lasts less than 15 seconds anyway so at the very least you’ll get upwards of 2 full runs between recharges.
ESC-550 and ESC-150
The inventor of the product has published a few dyno graphs demonstrating sustained boost levels of 5 PSI or more throughout the power band. Normally I wouldn’t trust a dyno graph as far as I could throw it. Everyone seems to pull these things out of their butts when it suits their advertising needs. However, when you consider that the ESC uses a conventional centrifugal supercharger design powered by AT LEAST 10 kW for small 4 cyls and up to 22 kW for V8 engines I’m inclined to believe the data. By the way, 22 kW is 30 HP just to run the blower and it all comes from spare batteries… 8 of them to be exact, fewer for the smaller models. These are small, high performance batteries as well so they don’t take up as much space as you might think and the total mass of the system is still comparable to a normal super or turbo setup.
An ESC-400 making 5 psi of sustained boost
Charging can be accomplished in a couple of ways either by charging at home or using a built-in parallel charging circuit that draws from your alternator as you drive normally to recharge the batteries. This takes power from the car but only after you’re done using it so it doesn’t matter unless you’re really uptight about your gas mileage while you’re charging the batteries. However you do it, the wiring is not included so you have to invest a little extra to connect everything and build your charging circuit if applicable.
The cost of the ESC starts at around $2000 and goes up from there. These prices are reasonable considering what aftermarket supers and turbos cost these days. You’ll easily pay $3000 or more for an equivalent belt/exhaust driven product. You usually get boost levels that are proportional to the higher price tag so in the end the ESC is fairly priced. On the other hand, we’re not just paying for the extra horsepower. The ESC is an electric solution, which gives it at least one clear advantage that I will elaborate on.
As I said earlier, ordinary supers and turbos draw power from the engine in order to operate. This increases the torque load on the engine. By doing so you burn more gas to overcome the resistance and you also increase stress on the engine. Aside from accelerating wear and tear, this extra stress means you have to be more conservative with your timing advance. Timing is increased to raise cylinder pressures and thermal efficiency. A stock engine can gain power just by advancing the timing and doing nothing else. You can’t go nuts but there is a little bit of free power waiting to be tapped by advancing the spark. If you add knock inhibitors to your gas or have a methanol injection kit you can maintain reasonable spark advance in boosted setups as well. The problem is, when the engine is under high load already it is less accommodating to timing advance. Advancing the angle (or not retarding it) can lead to dangerous knock that could ruin the engine. With an electric supercharger you don’t spend the engine’s power to make power. You spend auxiliary battery power instead so the crankshaft, connecting rods and pistons are not under the same high stresses, allowing you to get away with more spark advance safely with less wasted gas. This is a unique benefit of electric boost.
As of the time of this writing, the ESC by Thomas Knight is the only electric solution I’ve seen that’s capable of delivering on its claims and competing with older, well established technology in terms of gains per dollar. It is somewhat obscure but the science behind it is perfectly sound and the design is robust enough to be used in automotive applications without breaking on you.
Electrically supercharged Viper :)
There's an electric supercharger for motorcycles too.
So what’s the verdict?
Electric boost has been around for a little while now. The problem is, it’s still in its infancy. There aren’t any OEMs that have taken a stab at it and most of the aftermarket solutions are half-assed because they want to profit from the customer’s ignorance rather than make something that actually works. This has given electric boost a bad name, a name that it doesn’t deserve. There’s actually nothing wrong with electric boost. It’s a GOOD idea and a viable alternative to existing tech. It just needs time to gain a decent track record. It also needs skilled automotive enthusiasts to experiment with new designs so that the idea can really take off. Personally, I’d go electric because I know I can add more power afterwards with other mods like nitrous and the engine will cope longer before I’ve reached its full potential.