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