Saturday 21 February 2015

MX-5 Skyactiv-G Engines

Mazda MX-5 Roadster Skyactiv-G 1.5
The new MX-5 uses two variants of Mazda's Skyactiv-G direct injection petrol engine. There is a 1.5 litre with 130Bhp and a slightly larger and heavier 2.0 with 155Bhp. The 1.5 (P5-VP[RS]) engine is derived from that used in the Mazda 2 and 3 (Demio & Axela) and is said to be the purest MX-5 experience and most at home on twisty back roads, whereas the more torquey 2.0 (PE-VP) prefers the highway and high speed corners. The availability of the engines varies by market with some regions only getting one engine, and others seeing both.

These engines were specially tuned to be smoother revving, and create a nicer sound than the versions used in other Mazda models. Like the previous MZR L-series, the new engine uses an aluminium block and head, chain driven dual overhead camshafts, 4 pistons, 16 valves with continuously variable sequential timing (S-VT) on the exhaust and intake cams, counter-weighted forged steel crankshaft, plastic intake manifold, lightened flywheel, and a coil-on-plug ignition system.

ND's flywheel has been trimmed down

The new engines cylinders are designed with a smaller bore and longer stroke than the previous MZR unit, this allows them to be spaced closer together reducing the overall size of the engine. The larger 2.0 is structurally identical to the 1.5 just scaled-up proportionally thanks to Mazda's advanced CNC engine manufacturing process. This is true from the smallest 1.3 Skyactiv-G to the largest 2.5 version. The dimensions are different, but the structure, layout, and interfaces are all the same, which may present an opportunity for a future 2.5 engine swap. The recommended oil for these engines is a fully synthetic 0W20, but it's ok to use 5w30 or 5w20 too. The 0W20 is quite thin and helps reduce internal friction and further boost fuel economy. The 1.5 litre engine takes 4.1 litres, or 4.3 litres if you're changing the filter too.

The aluminium cam cover makes a return - good news for those who like to polish under the bonnet. Also returning is a feature previously seen on the NC - the induction sound enhancer (ISE). This extra pipe attaches to the intake is designed to further enhance the sporty sound the driver will hear, it will be standard on the higher trim levels.

There is a stainless steel 4-2-1 exhaust manifold with catalytic converter in a similar position to the NC. The reason for the change from the NC's 4-1 design is to help with knock prevention needed because of Skyactiv-G's high compression ratio. This version of the 4-2-1 is not as elaborate as those seen on other Mazda models due to space constraints. The new manifold looks a nice flowing design, but there will no doubt be lots of aftermarket de-cat 'performance' manifolds on offer in future.

Mazda's engineers experimented with at least 30 different prototype exhaust systems to achieve the right sound. Specifically studying the temperature distribution of the gases within the silencer and how that affects air density and the sound produced. The car had to make a suitably fun and sporty tone but also meet worldwide noise regulations. This work is discussed here by Yukifusa Hattori, the man responsible for the distinctive sound. His work almost makes you feel guilty for changing the intake and exhaust for aftermarket parts, knowing how much effort went into designing the originals.
The Skyactiv engine improves torque output despite doing away with the variable intake system used on the NC. For the 2.0 peak torque is 200Nm compared with 189Nm for the equivalent NC. This increase in torque combined with the lighter weight should make the car even more fun to drive. The exhaust pre-silencer and catalytic converter are precisely located to make best use of exhaust pressure waves in the system for maximum torque. An enormous amount of data was analysed to create the smoothest intake and exhaust flow whilst making a nice sound. Future aftermarket exhausts will have to be careful not to upset this fine balance and reduce performance.

To improve fuel efficiency these engines have been designed with a higher than normal compression ratio. This ratio describes the relationship between the size of the combustion chamber at it's largest (when the piston is at the bottom) to it's smallest size (piston at the top). A higher ratio means the engine is better at extracting energy from the combustion of the air/fuel mixture. This is a good thing, however you have to be careful as high compression engines are more likely to suffer from knock and detonation side effects.

Knock occurs when, due to excess heat, the air/fuel mixture ignites at the wrong time and causes the engine to run rough and can lead to engine failure. Modern cars are equipped with knock sensors to detect this and adjust the engine's timing to prevent it. The Skyactiv-G engine goes a step further and adds other features such as the longer than normal 4-2-1 exhaust header, specially designed cavity pistons, and direct fuel injection.

The new injection feature sprays very high pressure fuel directly into the combustion chamber using six-hole injectors for optimum fuel distribution. This helps to create a faster and more complete burn for efficiency and emissions benefits, whilst keeping temperatures under control. These advances have enabled the latest Mazda engines to generate more torque and use less fuel than previously possible.
To maintain the MX-5's signature 50:50 weight distribution, the engine is again mounted in a front-midship layout. Keeping the engine close to the centre of the car helps with handling as well as allowing a low swooping front end design. Both engines are mounted at a small angle off-centre, 6 degrees for the 1.5 and 7 degrees for the 2.0 due to the slightly larger size. Unlike many new cars, looking under the bonnet the engine is left largely on show. There is just a small cover piece across the inlet manifold featuring the "Skyactiv Technology" branding.

The i-Stop technology is another first on an MX-5. Mazda uses this to cut the engine when the car is stationery. The system determines the exact positioning of each piston and chooses the correct cylinder from which to restart the engine. The engine is then restarted by injecting fuel and igniting it, rather than constantly using the starter motor. The engine is said to restart in just 0.35 seconds and deliver around 8% better fuel economy as a result. The i-Stop feature can be turned off via a switch on the dashboard.
In the video below Dave Coleman explains how the Skyactiv approach influenced the design of the Mazda 3. He also talks in detail about the Skyactiv-G engine and offers some insight into it's performance. Also mentioned is the engine scaling which could have implications for those hoping to do an engine swap in future.

These two videos further explain Mazda's Skyactiv philosophy and how it applies to the new engines.

These driving footage videos give a good idea of the car's engine and exhaust sounds.

And here's a neat engine animation showing the direct injection and piston cavity.

Table of peak engine power (PS) by market

Engine Size USA Europe Australia Japan
1.5 - 131 @ 7000 131 131 @ 7000
2.0 157 160 @ 6000 160 -

Table of peak torque (Nm) by market

Engine Size USA Europe Australia Japan
1.5 - 150 @ 4800 150 @ 4800 150 @ 4800
2.0 200 @ 4600 200 @ 4600 200 @ 4600 -

Engine Specifications

Displacement (cc) 1496 1997
Bore x Stroke (mm) 74.5 x 85.8 83.5 x 91.2
Valves per Cylinder 4 4
Redline (rpm) 7500 6800
Compression Ratio 13:1 13:1
CO2 Emissions (g/km) 139 161
Economy (mpg) 47.1 40.9

Engine Bay Layout (LHD)
Mazda MX-5 Skyactiv Engine Bay
(1) ABS System (2) Windscreen Wash (3) Suspension Tower Brace (4) Air Conditioning

(5) Exhaust Heatshield (6) Intake Airbox (7) Coolant Filler Cap (8) Oil Filler Cap (9) Intake Manifold

(10) Electronic Throttle (11) Battery (12) Brake & Clutch Fluid (13) Main Fuse Box (14) Active Bonnet

Induction Sound Enhancer (ISE) pipework

The table below shows what is meant by the Skyactiv-G 1.5 engine being specially tuned for the Roadster. Comparing the specification to the 1.5 engine in the Mazda 3 (Axela) we can see there are quite a few differences. A larger throttle body, counter-weighted steel crankshaft, premium fuel map, and revised exhaust cam, combine to produce increased torque and peak power and a more sporty driving experience.