Mazda 13B-MSP ‘Renesis’ engine

Introduction

Mazda’s 13B-MSP (Multi Side Ports) ‘Renesis’ was a naturally aspirated, two-rotor engine with a capacity of 1308 cc, i.e. 654 cc for each rotor. The 13B-MSP engine was offered in standard and high-output versions that were used in the Mazda RX-8 from 2003 to 2011.

The name ‘Renesis’ was derived from a combination of Rotary Engine (RE) and Genesis.

Background: rotary engines

The main components of a rotary engine are the rotors, eccentric shaft, rotor housings, and the front, rear and intermediate housings. The rotor rotates within a rotor housing which is positioned between an intermediate housing on one side and the front or rear housing on the other to form a sealed chamber. In the rotary engine, each of the three faces of the rotor forms a chamber; the path of the rotor within the rotor housing is determined by the internal and stationary gears, which are phased, so that for every rotation of the rotor, the eccentric shaft rotates three times.

Like a four-stroke reciprocating engine, the air:fuel mixture is subjected to combustion and expansion. Whereas a reciprocating engine uses the vertical movement of the pistons and a valve mechanism to allow gas exchange, a rotary engine uses the rotation of the rotor to open and close intake and exhaust ports on either side of it. Since each rotor has three working chambers, every single rotation of the rotor produces three full working cycles, and each cycle has an intake, compression, combustion and exhaust stroke.

13B-MSP Renesis: side exhaust ports

In contrast to the Mazda RX-7’s 13B-REW engine which had one peripheral exhaust port per chamber in the rotor housing, the Mazda RX-8’s 13B-MSP Renesis engine had two exhaust ports per chamber in a side housing, alongside the intake ports. This configuration:

• Eliminated intake/exhaust port timing overlap to prevent the retention of exhaust gas, reduce gas flow resistance and promote stable combustion;
• Enabled the opening of the exhaust port to be delayed for a longer expansion cycle and increased thermal efficiency, power and fuel efficiency;
• Enabled unburned gases to be retained and burnt in the next combustion cycle. In contrast, the 13B-REW engine emitted unburnt gases from the combustion; and,
• Resulted in an almost 30 per cent larger cross-sectional area for the intake ports relative to the 13B-REW engine. Furthermore, the exhaust port area of the 13B-MSP Renesis engine was almost double that of the 13B-REW.

To minimise the accumulation of blow-by gases between the intake and exhaust ports, formed due to the slight gap between the oil seals and side seals on the side of the rotor, the 13B-MSP engine had an additional cut-off seal between the oil seals.

13B-MSP Renesis: side intake ports

The standard output 13B-MSP engine – offered with an automatic transmission – had two intake ports per rotor chamber for a total of four ports, with timing controlled by a variable intake valve. However, the high-output 13B-MSP engine – offered with a manual transmission – had three intake ports per rotor chamber (primary, secondary and auxiliary) for a total of six intake ports. Significantly, each port had different timing (see table below).

Sequential Dynamic Intake System (S-DAIS) and Fresh Air Duct (FAD)

The 13B-MSP Renesis engine had a ‘sequential dynamic intake system’ (S-DAIS) which, according to engine speed, controlled:

• The secondary and auxiliary ports; and,
• Opening/closing the variable intake valve (upstream of the secondary port’s shutter valve).

The operation of S-DAIS may be summarised as follows:

• At low engine speeds, only the primary intake was used;
• At around 3750rpm, the shutter valve would open, slowing intake flow and bringing the secondary port into operation;
• The high-output engine’s auxiliary port would open at about 6250rpm to maximise intake port area and boost high-end torque and power output; and,
• The variable intake valve would open at around 7250rpm (approximately 5750rpm for the four-port engine) to effectively lengthen the intake manifold for improved mid-range torque.

The high-output engine also had a variable fresh air duct (FAD) which opened a valve at around 5500 rpm to shorten the air intake manifold (upstream of the air cleaner) and operate in conjunction with the variable intake valve to increase power and torque at high engine speeds.

Injection and ignition

The primary intake port injector for the 13B-MSP Renesis engine was a 12-hole type, while the secondary and auxiliary (for the high-output engine) ports had four-hole injectors. Injection pressure was 392 kPa.

The ‘Jet Air-Fuel Mixing System’ enhanced the dispersion and mixing of fuel by utilising an air bleed in the intake port to increase air flow over the intake port walls and the atomization of fuel particles adhering to the port walls. The 13B-MSP Renesis engine had a compression ratio of 10.0:1.

Due to the large, bathtub shaped combustion chambers, the 13B-MSP engine had two iridium-tipped spark plugs (leading and trailing) per rotor housing to improve combustion efficiency. The leading and trailing spark plugs had different lengths such that incorrect fitment of the long spark plug could cause engine damage if it contacted the rotor.

The 13B-MSP Renesis engine had a hot wire flow volume meter with O2 sensors fore and aft of the catalytic converter in a double loop feedback control system.