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Efficient Engines 


Downsizing - smaller engines boosted with turbochargers - is not the only way to reduce fuel consumption and emissions

Nicodemo Angì

Since a smaller engine uses less fuel (besides being lighter and more economical) but also has less power than an engine of greater size, to obtain an effective combination between these characteristics, downsized turbo engines (already illustrated in a previous issue) have been created, with engine size around 1 litre and three cylinders capable of developing 100/120 hp.

Adding a turbocharger increases the costs (being a very sophisticated and expensive element), generates turbo-lag and creates an exhaust backpressure; it must be said, however, that electronically controlled geometry and other accessories allow reducing these drawbacks.

Four-stroke petrol engines have always been less efficient (compared to diesel engines) and this has prompted Toyota and Mazda to look for alternative measures.

Both manufacturers have in fact produced a series of engines without turbochargers but carefully refined to reduce fuel consumption and, at the same time, develop an adequate power output.

The overall performance depends on several factors, one of the most important being the internal combustion, an explosion that occurs between fuel (generally fossil) and an oxidizer (commonly air), moving the piston that turns the engine, generating power.



Performance constraints                                


Not all the Energy produced by an engine is transformed into movement, in fact some of it heats up pistons, cylinders, head and other parts of the engine that radiates, as if it were a heater, about 2% of the fuel energy.

The heat that is not transformed in mechanical energy must be eliminated: the cooling system serves this purpose.

A further passive element is represented by pumping, that is the energy wasted during air intake: an important advantage Diesel engines have is the absence of the butterfly valve, which reduces this type of loss.


Internal friction (between a piston and a cylinder or tappets that run on camshafts) also absorbs energy and generates heat, while other elements (water and oil pump, power steering, A/C system and alternator) also absorb power; therefore, more manufacturers tend to operate them electrically to reduce the load placed on the engine and consumption.

Mazda’s plans for its Skyactiv-G engine are focused on thermal efficiency – that is head, cylinders and timing – so as to reach a compression ratio of 14:1 ( a record for production engines ), and reach a high thermodynamic efficiency. The higher compression ratio must deal with that notorious knocking that, without a dedicated warning sensor, could damage the engine. But even if the engine is protected the torque output suffers because the engine control unit reduces the ignition timing to safeguard the engine.

Knocking is particularly sensitive to the temperature in the combustion chamber, both if the compression ratio is increased or if exhaust gasses are still in the chamber during intake.

Anti-knocking strategies developed by Mazda are aimed at reducing the temperature during compression.

The longer manifolds ensure that exhaust gasses from one cylinder do not go back into another through an overlap between inlet and exhaust valves.

The 4-in-2-in-1 manifold design has been studied to isolate, as much as possible, a cylinder in exhaust stroke from one in intake stroke.

Furthermore, fuel injection had to be optimized in order to have exhaust gasses hot enough to activate the catalyst since a long manifold tends to cool the gasses down.

This, though, causes unstable combustion; this problem was solved by adopting a piston cavity and optimizing fuel injection in order to formulate a stratified air-fuel mixture around the spark plug. Furthermore, the piston cavity resolved the issue of the initial flame coming into contact with the pistons head.



Higher compression, lower friction and fuel consumption!


Toyota, on the other hand, has worked much on improving the performance – especially relating to torque output – of its Atkinson Cycle engines, already used on hybrid models like Yaris, Prius and Auris.

The Atkinson Cycle is a variation of the Otto Cycle and presents compression and power strokes of different duration, with the second being longer than the first. This asymmetry is obtained through a particular timing that leaves the intake valves open during the compression stroke: air, therefore, is partially pushed back in the inlet manifold, reducing the “pumping work” other pistons have to perform. This allows increasing the compression ratio, something that we know has beneficial effects on the engines performance.

This expedient, though, reduces the torque output at low revs: hybrid vehicles can compensate this deficiency through the input of an electric engine, but not if the engine works on its own.

Therefore, to improve the output Toyota worked on a number of parameters, including the reduction of temperatures in the combustion chamber obtained through the liquid cooling of exhaust gasses recycled in the EGR system.

Fluid dynamics is of the highest level: the intake manifold impress a turbulent motion (tumble), which accelerates and enhances combustion, while a "wash" with pure air (scavenging) cleans the combustion chamber from exhaust gases, lowering the temperature.

Initially, these super efficient engines will come as a three cylinder 1 litre and a four cylinder 1.3 litre engines, in 14 different configurations: the 3 cylinder (developed with Daihatsu) has a compression ratio of 11,5:1 and scores a 37% in thermal efficiency while the 4 cylinder reaches a 38% efficiency thanks to a compression ratio of 13,5:1.

Further expedients (present only on the 1.3 litre engine) are an anti-friction treatment for the pistons as well as for the timing belt, special plastic covered bearings, the VVT-I system, low flow-rate oil pump (which absorbs less power) and the 4-in-2-in-1 exhaust system.

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