General Information
So, how does a rotary engine work, anyway?
There are some terms specific to the rotary engine that may help you understand its operation, or that you may want to refer to when viewing the table below.
Rotor:
A rotor is a somewhat triangular shaped engine component. It is roughly equivalent to the piston of a conventional engine, except that it has a total of three combustion surfaces (located between each apex) to the piston’s one (the top or face of the piston).
Apex:
Each rotor has three apexes, which are the points of the triangular shape of the rotor.
Eccentric Shaft:
The rotors drive the eccentric shaft, which is the equivalent of the crankshaft in a piston engine.
Rotor Housing:
A rotary engine consists of a sandwich with several layers. The rotor housing is one such layer that is the same width as, and contains a rotor. The inner shape of a rotor housing, which the rotor’s apexes follow, is called a peritrochoid curve. These housings contain the exhaust ports*.
Side Housing:
A side housing is another layer of a rotary engine sandwich that is much like the bread of a regular sandwich. Every rotary engine has exactly two of these as they are the layers that cap each end. These housings generally contain intake ports.
Intermediate Housing:
The intermediate housing is found between two rotor housings. Because the rotary engines found in RX-7s have two rotors, they have only one intermediate housing. Intermediate housings also contain intake ports*.
*Note: There are some rotary engines, called ‘peripheral port’ engines, that have their intake ports in the rotor housings and none in the side/intermediate housings. Mazda has reportedly developed a rotary with all side ports, including the exhaust ports, for use in the RX-01.
This photo is of a TKT Banzai 3 rotor engine prior to assembly. The front row, from the left, is the intake plenum, the two turbochargers, a side housing, a rotor housing, an intermediate housing specific to three rotor engines, another rotor housing, the intermediate housing common to two and three rotor engines, the last rotor housing, the other side housing, and the three rotors. Notice the exhaust ports in the rotor housings.
How the rotary engine works.
Other Comparisons to Piston Engines:
Displacement:
Rotary engine displacements seem small when compared to piston engines of similar power. In fact, both displacements are measured the same way. Displacement is the sum total of positive combustion chamber volume increases for one complete revolution of the main shaft (crank or eccentric). In a piston engine, this means the total amount of space swept by its pistons. In a rotary, it is easiest to think about the difference between the maximum and minimum volumes for a single chamber multiplied by the number of rotors (where each rotor has 3 chambers). Remember that the rotor actually revolves at one third the speed of the eccentric shaft, which is the reason only one chamber’s displacement is used in the calculation. The difference in power is due to the fact that the rotary uses its full displacement to produce power for each revolution of the eccentric shaft while only half the displacement of the piston engine is producing power for each revolution of the crankshaft. Other differences also play a role; rotaries do not have the losses of reciprocating motion and there is no valve train to power.
Combustion Frequency and Power Stroke Duration:
When you consider the facts above, you will see that on a rotary, each rotor fires once per eccentric shaft revolution. In a piston engine, only half of the combustion chambers fire for a given revolution. This means that a 2-rotor engine fires as often as a 4-cylinder engine. However, the power stroke duration in a rotary is 50% longer, it being 3/4 of a main shaft revolution to the piston engine’s 1/2. This makes a 2-rotor engine similar to a 6-cylinder.
Where does the turbo fit in?
Turbocharging is the exhaust-driven form of supercharging, wherein air is forced into the combustion chamber. When more air is available, more fuel may be burned, producing more power.
Mechanical supercharging involves a belt- (or sometimes gear-) driven air pump of one of two types, Roots or centrifugal.
The Roots type supercharger typically sits on top of a large V-8 engine and pumps air down into the intake manifold by the intermeshing of worm gears.
The centrifugal supercharger spins a fan-like blade to pump air through a pipe to the intake manifold. It’s placement options are more flexible so this type is more widely used.
The centrifugal supercharger may be driven by a belt (as described above), an electric motor (new technology), or by an exhaust driven turbine. This last form is turbocharging.
The Turbocharged Rotary Engine
In the above diagram you can see a series of turbine blades being propelled by the force of the exhaust gasses rushing out of the engine. These blades are connected, via a shaft, to a compressor which forces air into the intake via an intercooler. The intercooler is an air-to-air heat exchanger designed to cool the air which has been heated during the compression process. Cool air is denser than hot air and dense air is the goal of turbocharging.
Notice that the behavior of the intake airflow arrow differs in the turbocharged engine diagram from the normally aspirated engine previously shown. The size of the intake (and exhaust) airflow arrows signifies flow in volume and speed. In the normally aspirated engine, this is dependent on the vacuum created by the change in volume of the combustion chamber. Near the end of the intake “stroke” of the rotary engine, the volume of the combustion chamber nearly stops expanding, dramatically slowing the draw of air. In the turbochanged engine, the force of the turbo continues to ram air into the still open intake port, pressurizing the chamber with air, unlike the slight vacuum in the normally aspirated combustion chamber. With more air to mix with, more fuel may be added, and more power produced.
Prior to the turbocharger is the wastegate, a vacuum or spring held trap door which leads to a shortcut around the turbine half of the turbo. When turbo boost reaches a preset level, this door is gradually opened to bleed off the exhaust pressure, avoiding overboost. The diagram shows this wastegate in an open position.
When less power is needed, the turbine naturally ceases pressurizing the air and the combustion chamber’s vaccuum draws air in. Thus a turbocharged car can produce more power on demand without using more fuel under less demand.
How are the turbos configured?
The 3rd generation Mazda RX-7 has the world’s first production twin sequential turbocharged engine. The key word here is sequential. In every other automotive twin turbo setup, the turbos provide boost simultaneously. Each of the turbochargers in this type of application is generally smaller than the one turbo used in a single turbo setup. A small turbo accelerates quicker, suffering less from “turbo lag” than its larger counterpart and, as a result, produces less power and torque but sooner and at lower rpm. Fitting twin turbochargers in sequence produces better results as the first turbocharger receives the full force of all the exhaust gasses (instead of sharing with the other small turbo) and gains speed much quicker, which enhances throttle response and increases low speed torque. At a predetermined speed, the second turbocharger is called upon to add more boost. With the twin turbos in full operation, exhaust gas flow resistance is greatly reduced, contributing to higher power output.
Assuring a smooth transition from single to twin-turbo operation as been an inherent problem with the implementation of a sequential turbo system. If the secondary turbo is not spinning at a high enough speed when it is brought in, the whole system “staggers”, temporarily failing to produce enough torque for a smooth change-over.
Mazda’s rotary engineers attacked this problem with a vengeance and perfected a solution to this technical challenge. In the primary boost stage, when only he primary turbocharger is operating, a portion of the exhaust gas is led to the secondary turbocharger, spinning it into a “pre-operation” mode. The boosted air from the secondary turbo is not required at this stage, so it circulates in an essentially closed intake chamber. Left in this condition, the turbo would eventually go into what is called “surge”. This phenomenon is accompanied by a rapid temperature rise at the entry and exit of the compressor, which would harm the turbocharger if prolonged. In order to preclude this surging condition, a bypass valve is opened to form a loop in which the air circulates.
The secondary turbo maintains a pre-operation speed of around 100,000 rpm. However, this is still not high enough to effect a smooth transition to twin-turbo operation. The secondary turbocharger must accelerate faster. This is achieved by deliberately inducing surging by closing the bypass valve and letting the compressor spin within a closed chamber. This sends the secondary turbo’s speed to as high as 140,000 rpm. When this speed is attained, the secondary turbocharger receives its full share of exhaust gas, and, at the same time, a control valve opens, allowing the secondary turbocharger to start supplying boosted air, adding to the primary turbocharger’s. As previously stated, surging is harmful if prolonged, but in this transition state, it only lasts a few seconds, and therefore has not detrimental effect on the engine’s durability and reliability.
The RX-7′s 13B engine used twin Hitachi HT12 turbos with a 51 mm, 9 blade turbine and a 57 mm, 10 blade compressor. The turbine and compressor blades are a curved “high-flow” type that offers less resistance to air and gas flow. This results in faster turbine and compressor spin-up at high rpm.
The twin turbos are mounted on a cast iron exhaust manifold which has been named “Dynamic-Pressure” manifold by Mazda’s rotary engineers. This manifold is elaborately shaped to minimize the distance between the exhaust ports and the turbochargers’ entry paths, improving low speed boost by as much as 25 percent.
A special blueprinted, balanced, and contoured version of this same unit is used on our race cars. These units are capable of producing higher boost levels for extended periods.
Always remember to properly warm up and cool down your turbo and they will reward you with trouble-free operation.
What about handling?
Good balance is the key to great handling. Although the 3rd generation RX-7 is admittedly a good handling car, it is still a compromise. At Pettit, we seek perfection, and for this reason, we have developed a complete line of suspension components that allow you to fine tune your car’s suspension to your driving style. However, some tuning can be accomplished by changing alignment settings and tire pressures.
Alignment is critical on any car, especially on a performance car like the RX-7. We’ve seen new cars that are slightly out of specification, so it is a good idea to to have your alignment checked prior to high speed maneuvers or track events. The following recommendation come from our own testing, as well as from conversations with customers who compete in all different types of events. Remember, this is only a guide.
How to choose the right brake pads
Coefficient of friction:
A dimensionless indicator of the friction qualities of one material vs. another. A coefficient of 1.0 would be equal to 1g. The higher the coefficient, the greater the friction. Typical passenger car pad coefficients are in the neighborhood of 0.3 to 0.4. Racing pads are in the 0.5 to 0.6 range. With most pads the coefficient is temperature sensitive so claims that do not specify a temperature range should be viewed with suspicion. The optimum is to select a pad with a virtually constant but decreasing coefficient over the expected operating range of temperatures. As a result, the driver does not have to wait for the pad to heat up before it bites, and the pad fade will not be a factor so that modulation will be easy
Now that we have a foundation we can see that finding the pad of the right material and heat range affects your braking efficiency. You don’t want a race pad for the street, because you have to heat it up to its appropriate heat range before it bites. Not too far off from racing tires where the operating range is higher, so getting them to stick requires more heat.
The difference is here you pick a pad for your car based on driving habits, much like you would with tires.
If you are on the brakes non-stop and generating excessive amounts of heat then you want a pad and rotor combo designed to bite or grip at higher temps.
On the street we want bite right now thus a pad with a lower operating temp, and the trade off is fade at higher temps, (excessive braking or high speed braking) or reduced bite.
Hawk HPS Brake Pads:
The Hawk HPS Brake Pad is designed to provide you with advanced braking characteristics for the street. Some of the features of this pad are; extremely low brake dust and a high friction (torque) either hot or cold. They are virtually noise free and are very gentle on your rotors giving you a long brake life. This pad is designed for high performance street use and will provide the best combination of performance and reliability.
Hawk HP Plus Brake Pads:
The Hawk HP Plus Brake Pad can take the heat of the track, and still provide a good street able pad. This brake pad is designed for the serous street and autocross enthusiast. It features an extremely high friction output making it worthy of club racing and auto crossing. However, due to the dramatic friction levels produced by this brake pad in order to achieve “race level” braking: rotor wear, pad wear, noise and dust may be increased.
Hawk Blue (9012) Brake Pads:
Medium/High torque racing brake compound. They provide low pad and rotor wear with good modulation. Designed for road racing and rally applications where heat is between 250 and 1000 degrees Fahrenheit.
Cooling Upgrades
- Aluminum Air Separator Tank
- Aluminum Radiator
- Silicone Radiator Hoses
- Upgraded Cooling Fans
- 180° Thermostat
- 185° Fan Switch
Why do I need an AST?
The OEM Air Separator Tank (AST) is plastic made and prone to eventually fail. This can happen by a nearly invisible split at the seam of the tank or a deformation around the filler neck which reduces the sealing of the pressure cap. In either case, often unnoticed coolant leaking can lead to overheating and engine damage. The Pettit Aluminum AST is a direct replacement and upgrade for the stock unit and is far superior to the stock plastic unit. The Pettit Aluminum AST also includes a lever vent cap that allows you to safely depressurize the system for repairs or inspection.
Why do I need an aluminum radiator?
The RX-7 OEM Radiator is a quality part, many are still in use and working well after 16+ years. However, leaks are common from the plastic end tanks and any overheating will shorten the radiator’s life. An aluminum radiator from Fluidyne, Koyo or Mishimoto (just to name a few) can provide additional coolant capacity, increased cooling ability and a more robust aluminum design is less prone to leaking or damage caused to increased heat.
Installs in about 2 hours.
Why do I need silicone radiator hoses?
The factory rubber hoses from Mazda leave much to be desired because most hose leaks occur next to the clamps and prolonged heat can cause the rubber to swell and eventually fail. Cutting back a little hose and re-clamping will buy you some time against a major leak. However, the right choice would be to upgrade your stock hoses to a Pettit Racing HD Silicone Radiator Hose Kit. The kit includes both upper and lower radiator hoses and fresh clamps for a guaranteed fit.
Installs in about an hour – and a good excuse to flush / refill you coolant.
Why do I need upgraded radiator fans?
The OEM cooling fans and relays are another common problem. When they fail, temperatures and pressure in the cooling system raises until the radiator cap reaches it’s maximum pressure capacity and then leaks (spews) coolant. At that point continued operation can result in overheating and engine damage. There are four relays powering two fan motors which are controlled by the fan switch, A/C and ECU request (or manual override for testing). We commonly service and replace connectors, relays and motors due to their premature failing, but this is simply a band-aid. A better option is to buy Upgraded Cooling Fans. Upgraded Cooling Fans do away with the sub-par wiring and connectors in addition to increasing airflow and adding a level of durability you frankly don’t get with the OEM fans.
Why do I need a 185F fan switch?
The factory fan switch turns on the fans at 195F, with age these sensors become increasingly less accurate. We have seen on several RX-7′s temperatures rising past 205F before the stock fan switch triggers the fans. Our 185F fan switch bypasses the ECU and turns on the fans directly.
Why do I need a 180F thermostat?
The thermostat’s job is relatively simple but extremely important; it regulates the engine’s operating temperature. It does this by restricting the flow of coolant from the engine back to the radiator. The stock thermostat is set to open at 195° Fahrenheit, as it opens it begins to circulate coolant through the radiator. An Upgraded Thermostat will open at a lower temperature and begin cooling your engine sooner.
What Next?
If you’ve followed the Initial Cooling Upgrade path, and want reduce further intake temps, charge temps, under hood temps and heat soak – these next steps will help address the problem and minimize heat conditions experienced at track events or under heavy street use.
Pettit Racing calls it Cool Power, but you’ll call it magic. Pettit Racing Cool Power will allow your vehicle to perform to its full potential, with more power, efficiency and longevity. It doesn’t matter what other upgrades you already have (intercooler, exhaust, etc) it compliments and improves overall performance.
Pettit Racing Cool Power Kits use the latest proprietary technology for thermal protection. These materials do not emit or expose the driver to harmful substances. The Thermal Barrier Panel or “TBP” is made from non-woven thermal insulation and is created specifically for use in areas where minimum space is available. The Thermal Barrier Wrap or “TBW” is made of woven thermal insulation, both types resists temperatures up to 2000 degrees F. Both materials contain zero asbestos, are non flammable, will not corrode, and resist mildew and deterioration.
Cool Power Kits are available for Single or Twin Turbo vehicles and contain a number of items, including Thermal Barrier Wrap for exhaust components, Heat Shields for the lower intake manifold/turbo and Thermal Barrier Panels for induction and intercooler components. Used in conjunction, lost horsepower will be reclaimed, temps will drop and components will extend their longevity and reliability due to alleviated under hood temp conditions.