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08/09/2017
Too easy to call it combustion engine!

From engine to wheels

 

 

Waiting for the mass production of electric cars, the burden of reducing dangerous emissions and enhance efficiency will still fall on traditional combustion engines. A difficult task indeed, that is why “unconventional weapons” are being designed

Nicodemo Angì

 

Experts predict a booming electric car market able to trade millions of vehicles within a few years. However, for some time still, market shares are expected to remain quite limited, leaving traditional combustion engines with the task of becoming increasingly efficient and comply with emission standards. Success will largely depend on original solutions that will change the very nature of combustion engines.

Starting with Mazda’s proposal, which is currently looking at producing a petrol unit that works as a Diesel engine! The HCCI (Homogeneous Charge Compression Ignition) system, currently undergoing tests, eliminates spark plugs and modern stratified charge systems, that is, a small amount of a rich gas mixture that powers the remaining charge, which has much less fuel than the traditional air-fuel ratio of 1:14,7.

                                                   

Speeding up combustion                               

It may be useful to talk about flame speed, which is one of the factors that make petrol and diesel engines so different. The flame that ignites the combustion in a normal petrol engine comes from a spark plug at a speed of about 16 meters/second, which puts a limits to the size of the pistons used. Combustion only takes place when the flame is formed and additives are added to the petrol to increase the Octane content, which measures its ability not to ignite spontaneously. Even the compression ratio cannot go beyond certain limits without the risk of detonating. Which is precisely how a diesel engine works: its high compression ratio warms up the air so much that the fuel ignites all at once, producing the typical roughness of old school engines albeit with a high degree of efficiency. The modern Common Rail, thanks to its small multiple injections, is able to maintain the efficiency of the diesel cycle while improving its smoothness.

 

Spark plugs good bye!                                                   

Mazda’s idea for its HCCI engine (although the system is currently being studied by other manufacturers as well) provides for a uniform yet lean air/fuel mixture that is ignited not by an external agent (the spark plug) but spontaneously combusts thus overcoming the typical “slow” response of a spark-ignition engine. The matter, though, is far more complicated than this and the main problem lies with determining exactly when the combustion will take place: it will certainly take place after petrol is injected in the cylinder, but how can the exact timing be determined? Mazda came up with several solutions to establish exactly when the combustion takes place: a variable compression ratio, implemented with continuous timing and variable valve lift, intake air temperature control and the mixing of it with exhaust gases, acting both on the EGR valve and timing. The use of "hybrid" fuel is also evaluated, i.e. added with diesel or ethanol in real-time variable proportions. A rather complicated experiment for remarkable results: diesel performance, low operating temperatures, limited NOx production, and reduced CO2 emissions. A certain amount of COs and HCs will still be produced and will require emission control systems, but the benefits are still quite remarkable. 

 

2 or 4? Both!                                                       

Let us reclaim our spark plugs and use them in a hybrid unit, in which the combination is not between electric and combustion engines but between different cycles within the same engine. The idea came to Hyundai and stems from one of its patents filed in the US and describes a three-cylinder engine with two four-stroke side cylinders, while the middle one, with different bore, stroke, and timing works as a two-stroke. The patent describes two versions of the two-stroke cylinder, one with two valves and one with a more classical layout with a piston-controlled inlet port and open exhaust port; operation is modular until all three cylinders are working. No further details can be detected from the text and drawings, but let us remember that a 2-stroke cylinder, given the same engine revs will have twice as many combustions as a four-stroke. Couldn’t this cylinder then contribute to balance the engine? After all balance shafts offset second order vibrations rotating at twice the speed of the crankshaft and in the patent it is said that "noise, vibration and harshness (NVH) is further improved" compared to a classic 3-cylinder.

 

In no particolar order                                        

Still Hyundai submitted yet another patent application for a single engine with different cylinder displacement: 2 larger and 2 smaller. The reason was simply stated in an official release by the korean manufacturer: "Control margins for the operating point is insufficient due to the equal displacement of the various cylinders.  Furthermore, a lot of mechanical energy is wasted to guarantee a regular idling, and that is inefficient". Identical cylinders, useful for having a regular torque and a balanced engine, do not guarantee an efficient energy management. Hyundai is also looking at the use of an electric motor that not only gathers energy while braking and accelerating but regulates the idling when the working cylinders are “fewer”.

 

So many engines in one unit

“Fine-tuning” an engine as operating conditions change is pushed to extremes by Honda, after filing a patent for a 6-cylinder variable cylinder displacement engine with multiple stroke lengths on a single crankshaft. Since the bore is constant, the various cylinders would thus have different geometric characteristics, ranging from long stroke to a square and over-square bore / stroke ratio. The construction of such a unit might not be so complicated as the boring machine would be the same and the pistons could be identical (except for small modifications to vary the compression ratio). The electronic management of the engine, though, would be more complicated while balancing would be facilitated by the 6-cylinder layout. However, Honda has submitted patent applications also for 2 and 3 cylinder units of this type.

 

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