Will hybrids kill off the diesel tow kings?

We take three unusual suspects and ask will hybrids kill off the diesel tow kings – long live the towing hybrid?

Electrification will soon be big news for tow vehicles. Even Toyota has a petrol/electric hybrid powertrain to replace diesel in the next LandCruiser. Yet hybrid heavy-duty tow vehicle technology isn’t the way of the future – because it’s already arrived.

The Porsche Cayenne E-Hybrid is one of a growing list of heavy-duty hybrid tow haulers on the market today. But just how good is this brave new world of green-friendly powertrain compared with the best technology diesels available? To find out, we’ve hitched up a 2900kg Avida caravan behind the Cayenne E-Hybrid, the Audi’s SQ7 – with its super-tech triple turbo-diesel V8 – and to cap it off, the current Euro SUV benchmark, the VW Touareg 190 TDI.

SAME PLATFORM, DIFFERENT RESULTS

These three tow wagons share the same Volkswagen Group platform. Each has a similar eight-speed automatic transmission, and are all-wheel drive. However, they are not competitors: This is a comparison of technologies in the towing environment, not the vehicles themselves.

Getting into the latest powertrain technology isn’t cheap. The Porsche Cayenne E-Hybrid here with several options ticked (such as sunroof, LED lights and Bose sound system and of course the towbar kit – but not including fitting it) was $162,790 (plus on-roads).

The Audi SQ7 is a pricey top-shelf sports SUV wagon, so no surprise the vehicle tested here was $179,392 (plus on-road costs).

The Volkswagen is not pitched at the high-end premium SUV market, and therefore is priced at $80,790 (plus on-road costs).

TEST CRITERIA

The areas we were looking at included how the vehicles performed with a heavy van, such as ease of highway 100km/h touring, climbing hills and overtaking and engine braking downhill. The big question – just how fuel-efficient is a hybrid compared to diesel during heavy-hauling – was also a key datum. Fuel consumption would also in part dictate touring range, something for which many hybrids are already at a disadvantage, as their packaging (which has to incorporate a large battery and electric motor) often doesn’t allow for a large fuel tank.

Turbo-diesel technology hasn’t advanced much in the last 10 years, but we’ve brought along two relatively modern interpretations of the turbo-diesel to evaluate against the Cayenne E-Hybrid – a towing SUV with a high-performance bent, the Audi SQ7, and the SUV all-rounder, the Volkswagen Touareg 190 TDI.

The test involved towing a 2900kg Avida Topaz Multi-Terrain off-road van behind each of the vehicles on a 200km loop comprising a highway with a steep descent followed by a steep hill climb, an undulating freeway and then secondary roads with plenty of hill climbs. We could’ve found a big long flat piece of outback highway to cruise on, but that would not have put the vehicles under much duress. Our loop would be the acid test to see which technology can handle a hard-core towing environment.

AUDI SQ7

The Audi SQ7 was rolled out in 2016, so while it may not seem like the freshest in turbo-diesel tech, it is advanced. The 4.0-litre turbo-diesel appears quite conventional at its core, with high-pressure common-rail injection and two conventional exhaust-driven turbochargers. Where it gets special is with its ‘third’ turbo – an electric compressor that belts in compressed air to the induction system.

The compressor assists the two turbochargers whenever there’s a high load demand from the accelerator and insufficient boost from exhaust gases.

The SQ7 will accelerate to 100km/h in 4.9sec and achieves a claimed fuel average of 7.6L/100km. This is quick and efficient for a 2.4 tonne SUV.

The Audi has a 3500kg towing maximum with 350kg of that permitted on the towball.

The Audi employs a conventional eight-speed automatic and permanent all-wheel drive. Kerb weight is similar to the others here, at 2385kg.

Checking your mirrors should be routine when towing or not, but in the SQ7 you can’t help but re-double your mirror-checks. The thing is so damn quick and responsive at any speed, it’s hard to believe that it is actually towing anything at all. You just don’t expect the shove in the back the Audi V8 easily delivers while hauling a combination weighing nearly six tonnes.

The boosted V8 is really responsive. Just like when it’s unladen, there is no turbo lag of note – thanks to the peak 900Nm torque arriving at the doorstep at just 1000rpm and not leaving until 3250rpm. The Audi just gets up and gets going, up hill, down dale – and it sounds terrific while doing it.

The steep 80km/h climb on the test route was dispatched with not even half throttle input and when baulked by a semi, a few seconds of full throttle and it was up to 80km/h again easily and quickly. It’s almost unnatural how effortlessly this torque monster devours the road with a heavy van on the towball. Despite being such a free-revving engine, engine braking is pretty good – no doubt aided by the number of ratios you have to play with to peg speed.

Even though it’s a diesel, there are eight hungry cylinders to feed so the Audi’s consumption on this fairly demanding loop wasn’t stellar. The trip computer average climbed quickly to 15L/100km just 50km into the loop, and once we headed for the hills, it only increased. The average consumption on a tank-to-tank refill settled on 18.9L/100km. This is not bad at all when you consider the likes of a LandCruiser 200 Series, which would easily reach 20L/100km on such a route as we chose. With the Audi’s 85-litre fuel tank, however, you’d get not even quite 450km fuel range.

PORSCHE CAYENNE E-HYBRID

While the Audi and Volkswagen are not exactly simple vehicles, the Porsche is much more complex. We won’t get into all the E-Hybrid’s technical details here, only those relevant to towing. The E-Hybrid is what is known as a plug-in PHEV, which means it’s a petrol-electric hybrid that has a large battery that will accept charge from an external source such as domestic10amp 240-volt socket, or from electricity generated by the petrol engine or regenerative braking. It is driven by the petrol engine, the electric motor or a combination of both.

The engine is a twin-turbo 3.0-litre petrol V6 that develops 250kW. The engine is supplemented by a front-mounted 14.1kW/h electric motor that develops 100kW, making for 340kW maximum power on tap. While the petrol V6 develops 450Nm of peak torque, in combination with the 400Nm electric motor there’s 700Nm available (so 150Nm is lost when the petrol/electric torque is combined).

With its Li-ion battery (14.1kWh) fully charged, the E-Hybrid can travel up to a claimed 44km on battery power alone.

Just because the Porsche has a low emissions, high fuel economy bent, that doesn’t mean it is slow. The Cayenne’s performance/economy figures are impressive: a rest to 100km/h in 5.0sec and an official combined fuel consumption average (when not towing) of 3.4L/100km. Although the Cayenne can tow up to 3500kg (braked), it has a maximum towball download of 280kg.

The E-Hybrid has a variety of driving modes that offer everything from full petrol performance mode to electric-only driving. We set the Cayenne to Hybrid Auto, which uses satellite navigation information to combine petrol and electric motivation for the most efficient driving. We made sure that there was as much charge as possible in the Porsche’s EV battery at the start of the loop. It had a 40km EV range at kick-off.

Moving off with the van hitched, you’re faced with the strange sensation of not having an engine running. Instead, the Cayenne’s V6 is silent, with just the distant whirr of the electric motor heard. The engine soon cuts in though, and this it does regularly through the trip. In fact, while the Porsche’s engine has to work solidly to get the rig up hills (with another great soundtrack), in most downhill scenarios (unless the battery capacity has dropped too low) the V6 shuts down and you’re simply coasting, all the while the regeneration system trickling charge into the battery.

Throttle travel feels long because it seems like not much happens until you push it down to about half way. It’s designed that way as a fuel-saving method, so that there is a point of resistance you can feel to let you know that if you press beyond that the petrol engine will kick in. Yet even when taking off from a standing start just on the electric motor’s 400Nm (which happens even in Hybrid Auto, unless the battery has depleted), the Cayenne didn’t feel lacking in power. While the Audi might have its beefy 900Nm on the plate from 1250rpm, the Porsche has the electric motor’s 400Nm on tap from the the point it starts spinning.

Floor it though, and the full combined 350kW and 700Nm is on the menu, as we discovered on the long hill climb, the Cayenne expends little effort maintaining speed. Even with a corner requiring the rig to back off to about 65km/h, the speed could fairly quickly be restored to the posted 80km/h.

Engine braking was quite good, although on steep descents you’re very conscious that you’re relying on high revs in the lower gears to peg speed.

About 50km into the loop, the Cayenne was showing a fuel consumption figure of 11.5L/100km and the E-range (electric motor range) had gone from 40km at the start to just 12km. Not long after the electric range has disappeared to zero and the fuel consumption gradually climbed. We were left with a measured fuel consumption average of 17.6L/100km. With a 75-litre fuel tank, you’ve got about 425km of range before running out of premium unleaded.

Not all of us tow all the time, and this is where the Cayenne’s fuel consumption is even more impressive. Shorter (50km) journeys on the petrol V6 engine (only) saw an average of 8L/100km; with the EV battery charged to 40km range, in Hybrid Auto the Cayenne was showing a fuel consumption average of 1.7L/100km after 100km. By the journey’s end at 150km – the EV battery had depleted by about 120km – the Cayenne has consumed an average of 3.1L/100km.

VOLKSWAGEN TOUAREG 190 TDI

The Volkswagen’s V6 TDI engine has been around since at least 2010 in one form or another. This latest version, introduced in the third-gen Touareg last year, saw power increase 10kW to 190kW. While the Volkswagen’s peak power is a fair way off the 320kW and 350kW of the Audi and Porsche respectively, its 600Nm peak torque (available at 2250rpm) is getting close to the Porsche’s 700Nm.

It might lack the ultimate power of the others, but the Touareg is no slouch: Volkswagen claims the 190 TDI will reach 100km/h in 6.5sec. Combined fuel consumption is quoted as 7.4L/100km.

Like the Cayenne, the Touareg has a 280kg towball download limit with up to two occupants inside (reduced to 130kg when five occupants are in the car) but can tow a trailer weighing up to 3500kg (braked). The Touareg also has a fairly small fuel tank at 75 litres, although if you buy the 190 TDI Premium with the Sound & Comfort Pack you get a 90-litre fuel tank.

The Touareg suffers an annoying lag off the line when unladen, which becomes less pronounced with a caravan hitched up. The engine is really smooth, free-revving and quiet – there’s no gruff diesel grumblings going on here.

There was no issue keeping up 80km/h up the hill climb, and the Touareg was quite quick to rebuild speed after negotiating a tight bend on the climb. It had to work harder than the other two to get back up to speed, but it didn’t struggle doing so. While it had to work the transmission a little more than the others on the easy freeway cruise, the V6 TDI is a relaxed, torquey and smooth towing engine.

While the engine is a free-spinning unit, it doesn’t have the strong engine braking of older diesels. However, like the Audi and Porsche, with eight ratios you’re bound to find a gear that will help avoid the rig running away from you downhill – albeit sitting on high revs to do so.

The Touareg’s trip computer reached around the 11L/100km mark about 50km into the loop, and at the end was showing 16.3L/100km – and with the most accurate trip computer reading here, matching exactly our tank-to-tank measurements. This would see you tow for 460km before running out of diesel.

CONCLUSION

The Porsche Cayenne E-Hybrid shows that a hybrid can not only match performance of a good turbo-diesel when towing, but also match its economy. While certainly no fuel-sipping tree-hugger when lugging a van, the Cayenne E-Hybrid in this scenario is more fuel-efficient than a regular petrol V6. What makes it even more appealing is that not only does hybrid technology work in heavy towing, but it works even better than diesel could possibly do when not towing.

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