Is a small-ish 4-cylinder turbo diesel engine suited to a large SUV?

Is a 3-litre 6-cylinder diesel or proper turbo V8 muscle the right engine options for the modern large SUV?  With the introduction of smaller yet more powerful diesel turbo engines, Mercedes-Benz thought the time is right to introduce the ML250 BlueTec – it used a 2.1-litre 4-cylinder turbo diesel with outputs of 500Nm (torque is more important here) and 150kW.

Mercedes-Benz GLE, Volvo XC90 and now BMW X5 – the smaller-engined cheaper big SUVs

The Mercedes-Benz ML has since been replaced by the recent ML facelift & rename, swapping Mercedes ML (a.k.a. M-Class) for its repositioned Mercedes GLE name.  It is available in GLE250d 4Matic (4-wheel-drive) using the same small-for-its-body-size 2.1TD engine with 500Nm/150kW, but the upgrade to the new 9-speed Mercedes 9G-Tronic gearbox has seen performance and economy improve, for acceleration of 8.6 seconds and consumption in the combined cycle down from 6.5 to 5.9 l/100km, giving a driving range of about 1500 km on a full tank.  The Mercedes-Benz GLE250d 4Matic new price is R 923 400 plus emissions tax, and is the entry-level model in the GLE range.

Volvo introduced its new XC90 to a long-awaited South African market in 2014, and it will only be offered with 2.0-litre 4-cylinder engines – all of them – whether petrol or diesel.  The XC90 D4 is the starting point for the new Volvo XC90 range, being even more brave than Mercedes by fitting a 2.0TD giving lower 400Nm and 140kW, though it is front-wheel drive rather than 4-wheel, for an even better claimed 5.2 l/100km / 136 g/km, average.  Its price, as entry-level XC90 model, is R 841 800 for the XC90 D4 Momentum.  The engine is evidently tuned for economy rather than performance, with the big Volvo taking a rather sluggish 9.2 seconds to reach 100km/h.

Move up to the Volvo XC90 D5 AWD, and the still-a-2-litre diesel improves to 7.8 seconds, thanks to an increase to 470Nm of torque and 165kW, possibly helped by the AWD 4-wheel-grip, while consumption is still a very low 5.8 and emissions of 152.  Its price starts at R 890 900.

BMW has just added a new BMW X5 xDrive25d to bring in the X5 at a lower price-point, making R 889 668 the most-affordable new BMW X5 price on the new-car market.

It also uses a 2.0 turbo diesel engine, though outdoes the 2 main rivals by offering a 170kW power max, and a Merc-matching 500Nm.  The BMW X5 xDrive25d is only available with xDrive 4-wheel-drive.  The official BMW acceleration is given as 7.7 seconds, while the 5.6 and 146 (l/100km and g/km) claims it to be the best in this group of 4-cylinder diesel 4-wheel-drive SUVs. 

BMW X5 xDrive25d versus Volvo XC90 D5 AWD

The closest near-identical rivals are therefore the BMW X5 xDrive25d and Volvo XC90 D5 AWD:

BMW X5 xDrive25d	Volvo XC90 D5 AWD
2.0 4-cyl turbo diesel	2.0 4-cyl turbo diesel
8-speed Steptronic auto	8-speed Geartronic auto
170kW	165kW
500Nm	470Nm
7.7 sec 0-100	7.8 sec 0-100
5.6 l/100km	5.8 l/100km
146 g/km	152 g/km
R 889 668 (base spec)	R 890 900 (base spec Momentum)

 
 So, is the small engine enough for the big SUV?

It seems so – the benefits of a lower purchase price and the promise of lower fuel consumption than its 6-cylinder diesel siblings make a business case.  The only negatives are the rough sound and some vibration felt in a 4-cylinder diesel – yes, even the new BMW and the Volvo XC90 are not as smooth as petrol engines nor the bigger 6-cylinder diesels.

If you want the smoothness and often-enjoyable sound of a 6-cylinder diesel thrum plus that mid-range torque thrust, only the bigger diesels will do.  If you want to save on price and fuel, the 4-cylinder diesel will (have to) do.

The Mercedes-Benz GLE and BMW X5 are available with a choice of either 4 or 6-cyl (the BMW X5 offers 30d, 40d and M50d models of the 3.0 6 diesel), but not the Volvo XC90.