How does a turbo engine affect fuel efficiency?

By Martin Pretorius

Modern legislation forces manufacturers to employ innovative approaches to boost the fuel efficiency of their offerings: there are rules which dictate how much fuel a vehicle should use, and there is a tax which penalises buyers of cars with high carbon emissions. One of the solutions aimed at meeting the conflicting requirements of low consumption, reduced emissions, and an ever-growing demand for power, is the trend towards downsizing.

What is downsizing?

It simply refers to the philosophy of using smaller engines instead of big ones, which are then massaged to produce similar power to their bigger counterparts. One of these “massage techniques” involves forced induction, and it usually uses a turbocharger to force air into the engine. The theory is simple: smaller engines use less fuel and produces less tailpipe emissions, which helps the manufacturer to meet their legal requirements.

Unfortunately, smaller engines also produce less power, but this problem is overcome by forcing more air into the engine by means of the turbo's action. More air going into the engine means that more fuel can be added to that air, which means more power coming out of the engine. This way, the engine can use less fuel while it's being tested in a laboratory, but it can still deliver the on-the-road power demanded by the buyers.

Good idea, but does it work?

To understand the downsides and advantages of using a turbo to boost the power output of a small engine, it pays to examine the legislated testing procedures. To a large degree, these tests are performed at very low power levels, which means that a smaller engine will naturally perform better than a large engine – there's usually enough power available to perform the test, without even making use of the turbo's ability to boost the power.

As a result, downsized engines perform well in these standardised tests. Unfortunately, real-world driving conditions are a lot more demanding than the consumption- and emissions tests, leading the turbo to come into play to increase the available power.

Here's the problem: to make a car move at a certain speed, that car would need a certain amount of power. That power is generated by burning a certain amount of fuel – and the fuel doesn't care whether it's burning in a small engine or a large one. The more power you require, the more fuel you will need to burn, be it in a large engine or a small one.

Tuning comes into play as well.

This article could drown in technicalities right about now, so in the interest of keeping things simple, we won't explore the various adjustments regarding the ratio of fuel and air going into the engine under different load conditions. Just know that a turbo engine will invariably need some extra fuel (use a “richer” air-fuel mixture) when the turbo is delivering pressurised air, which adds a slight consumption penalty to the turbo engine's operation.

Let's translate that: a boosted engine requires a richer mixture under higher load conditions, in the interest of keeping combustion temperatures down and preserving the engine's life. And this is where the theory of downsizing falls somewhat flat: yes, smaller engines need less fuel than large engines when they're not working hard, but they need more fuel when their turbos force them to deliver more power.

Is it worth the hassle and complexity?

If the engine can deliver enough power to cruise at freeway speeds without bringing the turbo into action, that requirement for extra fuel when boosting obviously doesn't come into play, and in these conditions, the smaller turbo engine will use less fuel. But when the small turbo engine is asked to perform the work of a bigger one when the driver demands power, it may well use more fuel (for the duration of that power demand) than a larger, non-turbo engine of equal power output would use.

It's a compromise: more frugal cruising with the penalty of higher consumption under hard acceleration with the turbo engine, or thirstier cruising but greater frugality when working hard with the larger, equally-powerful non-turbo engine. It depends on the application, really – but for most drivers, the smaller, turbo engine will be just fine. The roads are too crowded for hard acceleration or high speeds, anyway...