The first time I ever heard of twin charging (using both a turbocharger and a supercharger on the same motor) was probably back in year 2000. At that time I was very interested in performance for the Toyota Celica and naturally I also read a lot about its sister cars (that shared some of the same engines) such as the Camry and the MR2.
One of the most interesting aftermarket parts I ran across at the time was the HKS turbo kit for the 4AGZE powered 1st generation mr2. The 4agze (for those that are not familiar with Toyota engines) is a peppy 170 horsepower 1.6 liter engine powered by the Toyota SC-12 roots type supercharger. On this car Toyota used an electromagnetically clutched supercharger that could be disabled during low power requirements such as cruising, and engaged when the user demands it.
One of the most important parts of the HKS kit is the bypass valve. This valve was used to direct air from the supercharger to the engine at lower rpm/flow points. Once the rpm’s rise, and the engine starts to demand more air, and the turbocharger is fully spooled, the valve switches over gradually till the turbocharger alone is feeding the engine while the supercharger is completely bypassed. The twin-charged MR2’s were rumored to break the 300hp mark in some cases, depending on the final boost level and the supporting modifications, and this level of power for a 1.6 litre motor at the time was quiet astounding.
The theory behind this kind of system is to use a small positive displacement (roots style) supercharger. Supercharger performance efficiency is typically at its highest at lower engine and supercharger rpm’s (for example from idle to 4000 rpm’s). Above 4000 rpm’s the supercharger’s performance and efficiency starts to drop, the horsepower required to drive it starts to rise exponentially, and the air temperature coming out of the supercharger starts to rise dramatically limiting performance.
On the other hand, using a generously sized turbocharger will allow us to feed the engine efficiently with cooler air (than that from an overworked supercharger) and maintain high rpm performance. The problem with using a larger turbocharger is that a generously sized turbocharger typically doesn’t spool before 3000 to 4000 rpm’s giving us a limited power band and thus providing no performance boost at lower rpm’s.
The idea of twin charging is to use both a supercharger and a turbocharger to have each charger do what it does best, have the supercharger boost the motor for low end torque, and as it runs out of steam, the turbocharger comes online to carry us through to redline.
There are three aspects to these types of systems that make them prohibitive to most tuners:
1. Cost and complexity: Having a complete supercharger system as well as a complete turbocharger system on the same vehicle is a lot of money to spend and a lot of parts to deal with and diagnose in case something does go wrong.
2. The bypass valve used to bypass the supercharger (and yet hold in all the air pressure coming from the turbocharger) as well as being able to control this valve electrically or mechanically requires a custom made one off valve that isn’t quite available off the shelf. Although as I write this it seems possible to find a large sized dual chamber bypass valve plumbed to operate on the differential pressure between the turbo outlet and the supercharger outlet to switchover once the turbocharger pressure = the supercharger pressure + the tension of the bypass valve opening mechanism.
3. Since we are using two different types of chargers with two different efficiency maps, it can get very complicated to figure out how to tune the motor (especially with much simpler fuel injection systems that were used at the time) because the air density can vary dramatically at the same rpm point and pressure level depending on which charger is feeding air to the motor and at what proportion. This is also where the HKS turbo kit for the 4agze was at its weakest, namely at smoothing the transition point fueling between the supercharger to turbocharger switchover.
One of the things that has changed over the last 10 years is the availability (and proliferation of knowledge) about available alternative fuels or octane boosters. Two such options are:
1- E85 fuel which is comprised of 85% Ethanol which has an octane rating of about 100 to 105 octane vs the typical 87 to 93 octane pump gasoline.
2- Water / methanol injection systems that can be used either as supplemental fueling system (based on the methanol content which carries an octane rating of 110 octane or higher) or can be used for in cylinder cooling when the water vapor injected with the methanol transforms into steam inside the combustion chamber, thus extracting lots heat out of the combustion chamber, and thus slowing down the speed of travel of the combustion flame front simulating the effects similar to those of a higher octane gasoline.
With the availability of these octane increasing or octane simulating concoctions, it has become more accessible of recent for the performance enthusiast to build a different type of twin charger system that does not require a bypass valve.
In this type of system the supercharger outlet is routed to feed the turbocharger inlet or vice versa. Rather than either the supercharger or the turbocharger feeding the engine individually (in parallel operation) and switching between the two, we are now using a two stage compression system where one stage is the factory supercharger, and the 2nd stage is an aftermarket turbocharger system.
The net result of the two compressors is a compounding of pressure ratios. For example if the turbocharger waste-gate opening spring is set to a setting of 7psi of pressure above atmosphere (which is a pressure ratio of 1.5 given that 1 atmosphere is about 14.7 psig); and if the supercharger is mechanically geared to flow 50% more than the engine (for positive displacement roots style superchargers) at any rpm, thus having an identical 7psi boost setting or a pressure ratio of 1.5; then the resultant pressure ratio of the system combined is :
PR total = PR turbo * PR supercharger = a pressure ratio of 2.25
A pressure ratio of 2.25 is equivalent to 18.4 psi of boost (not 14psi expected by adding the two stages together).
So anyway, how does this relate to octane requirements ?
If the turbocharger is feeding the supercharger for example, and the turbocharger is ingesting fresh air at ambient air temperatures (T1), then:
1- The air exiting the turbocharger will be at a temperature T2, higher than the ambient air temperature (T1) by about 60-80*C depending on the exact turbocharger, and where we are on the turbocharger compressor and efficiency map.
2- The air entering the supercharger will enter at a temperature T2 ~=T1+60 and exit at a temperature T3 which is higher than T2 by about another 60-80*C depending on the exact specifications of the supercharger.
3- If we had an intercooler after the supercharger, then the air entering the intercooler will be at 120 to 160*C above ambient temperatures which is a lot of heat for the intercooler to attempt to shed in the short amount of time that the air passes through the intercooler core.
4- If we have no post supercharger intercooler (which is common on cars where the supercharger is packaged into the intake manifold of the car), then the air entering the engine will be at some 120 to 160*C above ambient.
5- This excessively heated air not only reduces power output (By about 1 horsepower for every 13*C) but it also increases the probability of the air fuel mixture automatically igniting in the motor pre-maturely before the spark plug has fired, and if this pre-mature ignition occurs early enough to catch the piston significantly far away from top dead center, then the battling flame front pushing the piston downwards, and the inertia of the system (and force of other firing cylinders rotating this piston via the crankshaft) pushing the piston upwards will cause extremely high pressures and a temperature rise on the surface of the piston ultimately damaging it and possibly damaging other parts of the motor as well.
For these reasons (pressure compounding, and combined temperature rise) sequential charging has seen very little application in the past. The use of a higher octane fuel by definition means that the air fuel mixture is more resilient to auto-ignition and detonation. Furthermore, in the event of a pre-mature ignition, the higher octane fuel creates a slower traveling flame front which gives the piston more time to travel upwards in the cylinder bore (Closer to top dead center) before meeting the flame front and this reduces the time that the piston surface is improperly pressurized and overheated reducing the possibility of catastrophic failure. Last but not least, the use a water / methanol injection mix includes two phase-change events:
1- The injected methanol changes from a liquid state to a vapor state at its boiling point of 65*C, i.e. as soon as it hits the compressed air mixture coming from the supercharger outlet. This phase change absorbs a lot of the heat out of the air and methanol mixture reducing inlet air temperatures even before the mixture reaches the combustion chamber and starts to get compressed. This temperature reduction goes a long way towards eliminating or highly reducing the possibility of detonation.
2- The injected water, changes from a liquid state to a vapor state at its boiling point of 100*C which depending on the availability of an intercooler in the system, my occur in the intake plumbing before reaching the combustion chamber, or may not occur until the mixture is ignited. Either way, when the temperature is high enough, the water mist injected in the air stream will flash vaporize into steam also absorbing a generous amount of the heat created in the combustion.
The availability of these two octane boosters makes it now possible for aftermarket performance part manufacturers to deliver safe and reliable sequential charging kits to the mass market.
One such kit which I ran across in an article from hot rod magazine was developed by hellion performance (http://www.hellionpowersystems.com) for the factory supercharged GT-500 mustang.
The kit supposedly produce up to 1000 horsepower at a boost level of 24 psi using two 61mm Turbonetics turbochargers.
To achieve 1000 hp requires around 1500 cfm of airflow at 24psi or 1500cfm at a pressure ratio of 2.63, or 750cfm @ 2.63pr per turbocharger.
Since most compressor maps for this size of turbocharger (61mm) peak out at around 600cfm @ 2.63 pr @ around 50% efficiency which is an extreme point on the map (i.e. the turbocharger is maxed out at this point). I’m going to say that I am confident that the kit is capable of supporting 800hp with a typical pair 61mm turbocharger, however 1000hp although dyno-proven, does not agree with what is published on most 61mm turbochargers. I’m not doubting the kit, I am stating that I don’t have a better reference for the specific turbocharger used in the kit.
Furthermore, feeding 1000hp from 8 injectors requires eight 750cc/min injectors by my estimate and this agrees with what is mentioned on Hot Rod magazine’s article of needing 75lbs/hour injectors (each lb/hour is roughly equivalent to 10.5cc/min) at a minimum or a total fuel deliver requirement of 900 liters per hour of fuel at a the fuel rail pressure which is typically around 45psi.
Looking at the flow capacity of the GS342 fuel pump supplied with the kit, which is 255lph @ 30psi, then using 3 fuel pumps gives us the capacity for 765lph which is about 2125 hp worth of fuel, so in that regard the kit is capable of supporting the power figure.
As you can see, it is possible to design such a complex system if the information (Turbocharger compressor map, turbocharger temperature map, supercharger compressor map, supercharger temperature map …etc) information were available before hand. What remains a mystery and an art of trial and failure, is how over-engineered is your engine, how much torque can it produce and still continue to survive, and how long can it continue to survive at elevated power levels. That is altogether a more exciting question to answer.