Small Turbo Engines: Is The Hype Over?

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The claimed average fuel consumption/emissions figures of contemporary vehicles are calculated in a way to ensure that they will comply with tightening emissions regulations, not give consumers realistic indications of how much fuel their cars should consume in general use. Therefore, downsizing (fitting smaller, turbocharged engines augmented by turbocharging) is not an ideal solution to better fuel economy – and the emissions game's about to change anyway.      

Downsizing refers to the process of either reducing an engine’s cubic capacity or reducing the number of cylinders. Most manufacturers combine both techniques in order to reduce fuel consumption. This will also reduce CO2 emissions because when consumption in L/100 km is multiplied by 23.3 for petrol engines and 26.2 for diesel engines the answer will be the CO2 values in grams/km.

Some V8 engines have been dropped in favour of 6-cylinder engines, while sixes have been replaced by fours. Many naturally aspirated four-cylinder engines with capacities between 1.4- and 1.6-litre have been replaced by turbocharged 3-cylinder engines displacing between 1.0- and 1.2-litres. These changes have been forced onto manufacturers by the ever more stringent fuel consumption limits demanded by governments.

A really good video showing the components and operation of a three-cylinder engine.

Advantages

Removing 1 cylinder from a 4-cylinder engine, or reducing the cubic capacity by a significant amount must have the following advantages:

  • Internal frictional losses will be reduced since the piston ring/bore interface is the highest source of friction in an engine.

  • The engine will weigh less, take up less space, and will be cheaper to produce in unblown form. If it is turbocharged the price advantage will be lost.

  • When it comes to engine repair time the smaller number of cylinders will be a distinct advantage.

  • Driving sedately MAY result in low fuel consumption values.

  • You may end up paying a zero CO2 emission levy. Passenger vehicles emitting more than 120 g/km of CO2 are taxed at R75 per g/km for every km above 120 kph when buying a new vehicle.   

Disadvantages

  • Many downsized engines are turbocharged or supercharged to restore the power and torque lost due to smaller displacement. This will increase the complexity and service costs of the power unit as well as the control mechanisms.  

  • Road tests by magazines and motoring organizations have shown that most downsized engines, and especially the smaller ones, find it difficult to achieve the claimed fuel consumption figures.

  • Some downsized engines are not as smooth as the bigger units. The smaller number of cylinders always introduce higher levels of vibration that often require special mechanisms to reduce the vibration’s amplitude. For example, 3-cylinder units introduce a transverse torque that tries to tilt the crankshaft (when seen from the side of the engine). This has traditionally been balanced by an engine-speed balance shaft, and the threes from BMW, GM, Fiat and Nissan do incorporate such as shaft. However, the new Ford and Toyota 3 employ an innovative setup using specially unbalanced flywheels and front pulleys to achieve the required engine smoothness. This is an extra complication compared to an old-fashioned 4-pot, but modern 4 cylinders often have balance shafts as well.

  • Small turbocharged engines tend to suffer from turbo-lag. Such an engine needs time to spool up to the high (up to 250 000) revs where it delivers a high boost. BMW get over this by employing a small low inertia turbo for low-speed boost and a larger turbo for high-speed boost. Volvo uses  employs a supercharger to enhance boost at low speed and a turbo to deliver extra boost a high speed.

Why the heavier fuel consumption?  

  • A major reason for being disappointed with the fuel consumption is that motorists expect to get close to the manufacturer’s published figures. These are usually measured according to the NEDC (New European Driving Cycle), and this test is known to be unrealistic.

  • During full-throttle acceleration in a petrol-engined turbocharged or supercharged car there’s always a risk of harmful detonation taking place. The control unit copes with this risk by either retarding the ignition timing or supplying a rich fuel mixture or both, as soon as the knock sensor gives a warning signal. In the first case the engine deviates from the ideal timing and in the second case the fuel consumption increases.

  • Diesel engines do not have a detonation problem, but runs very lean at part throttle. This means that they also tend to use more fuel at large throttle openings.           

  • Blown engines usually run at significantly lower compression ratios than unblown engines to avoid the risk of detonation that arises from the increased combustion pressure and temperature. This lowers engine efficiency.

  • Driving sedately is difficult with a turbo because it’s natural to use extra power if it is available.  

The above paragraphs show that one can only get good fuel consumption from a turbocharged engine by avoiding large throttle openings.

NEDC (New European Driving Cycle)

Measuring fuel consumption on the road does not give repeatable results. There are too many variables that cannot be controlled. This test was developed in the late fifties and updated in 1970 to provide a more scientific way to measure fuel consumption. The test is done in a temperature-controlled room on a chassis dynamometer in a strict sequence of events to reduce uncontrollable influences. Unfortunately the test is completely out of sync with modern motoring habits. One can see from the graph and the video that the vehicle speed is mostly below 120 kph and the acceleration is leisurely. In a turbocharged engine this means the turbo is just idling so that its above-mentioned disadvantages are masked.

A Dutch video with English subtitles. It shows a journalist driving on the road according to the NEDC test cycle in order to demonstrate how ridiculous the test is. The NEDC is composed of two parts: ECE-15 (Urban Driving Cycle), repeated 4 times, is plotted from 0 seconds to 780 seconds; EUDC (Extra-Urban Driving Cycle) is plotted from 780 seconds to 1 180 seconds.

Summary

Modern engines (especially 3-cylinder turbocharged ones) are not designed for the real world; they’re designed for the NEDC and similar Japanese and American test cycles, but a change is on the way. The Volkswagen emission debacle has put a lot of pressure on the industry to adopt a more realistic on-the-road test cycle. 

The European Union is introducing new-model on-the-road testing for NOX pollution next year, and for fuel consumption and CO2 emissions 2 years later. This announcement has forced most automotive engineers to admit that their cars emit far more pollutants on the road than on a NEDC dynamometer test.

The surprising consequence is that some manufacturers are having second thoughts about extreme downsizing and fitting turbos. At the recent Paris motor show Thomas Weber, head of research and development at Mercedes, said: “It becomes apparent that a small engine is not an advantage. That’s why we didn’t jump on the three-cylinder engine trend.”

What's more, Reuters reports that the new test will effectively kill the development of small turbocharged engines due to the fuel consumption penalty (for petrol engines) and NO X penalty (for diesel engines) when driven at real-world speeds.

The agency quotes sources that suggest "Renault, General Motors and VW are preparing to enlarge or scrap some of their best-selling small car engines over the next three years" and "other manufacturers are expected to follow".

"The tougher tests may kill turbodiesel engines smaller than 1.5-litres and (petrol motors) below about 1.2-litres, analysts predict", the report adds. 

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