Toyota Prius C Vs Hyundai i30: Diesel vs hybrid – which is more fuel efficient?
If you really want to save fuel which is best, petrol/electric hybrid or turbo-diesel? We compare the 2015 Toyota Prius C and 2015 Hyundai i30.
ON THE LEFT IS Toyota’s small hybrid, the Prius C (review can be found here). On the right is our long-term tester, the Hyundai i30 Active Diesel automatic. Both represent the latest technology – the Prius has been in production for 18 years now, and the Hyundai diesel has a modern common-rail injection system allied to a seven-speed DSG gearbox (which Hyundai designed and built) which not only has more gears than older systems, but is more efficient, too.
Let the battle commence…
Here’s the vital statistics of each vehicle:
|Car||Engine||Output||Transmission||Weight||Length / width / height|
ADR81/02 Fuel Consumption
Urban / Extra Urban / Combined
Emissions of Co2 (grams/km)
|Driveaway cost (Melbourne)|
|Hyundai i30 Active Diesel||1.6L turbo diesel||100kW @ 4000rpm|
300Nm @ 1750-2500rpm
|7-speed DSG||1400kg||4300 / 1790 / 1470|
5.9 / 4.3 / 4.9
|Toyota Prius C||1.5L petrol & electric|
54kW @ 4800rpm
45kW / 169Nm
Combined: 74kW / 280Nm
|CVT||1140kg||3995 / 1695 / 1455|
3.7 / 3.8 / 3.9
It’s not actually a truly fair comparison because the i30 is larger, over 300mm longer and 95mm wider. That accounts for its extra weight, some 260kg, but diesels are also heavy engines, even if the Prius has to cart around a battery.
The official fuel consumption figures are interesting. The Urban cycle usually has the highest consumption because it’s stop-start which involves lots of accelerating, and the Extra Urban the least because the car is mostly cruising, with the Combined somewhere between the two. The i30 follows this pattern, but the Prius does not. This is because once in cruise it cannot leverage its electric motor which, along with the battery, becomes dead weight. We have more on the format of the official fuel tests here.
You can compare Australian vehicles for fuel consumption and emissions over at the government’s Green Vehicle Guide. Here’s the results for our two cars:
The emissions Co2 figure is how much carbon dixoide, in grams, is emitted during fuel consumption testing. Petrol and diesel engines perform quite differently in this respect and there is no easy soundbite to answer which is better or worse.
In brief, diesel engines use less fuel per kilometer than petrol, so advantage diesel. If we then look at the various types of pollutant emitted by internal combustion engines (both petrol and diesel) then diesels do better than petrol, except in the case of particulate matter (tiny particles of soot) and nitrogen oxides (NOx) emissions which “can cause a range of adverse health effects” according to the Guide.
However, every new generation of diesels is improving (as indeed are petrols). The latest Euro 5 standards are particularly tough, as are California’s emissions laws. This is why you see only modest power and torque improvements from new engines, because all the development effort is going into making the engine clean which tends to reduce power and torque. So in many ways if the carmakers can get the same engine outputs from a new, clean engine as the older, dirtier one that’s a win.
About the Prius…
The Prius is a hybrid petrol-electric. This means it has two engines, one petrol, one electric. The basic principle is that the car converts what would have been wasted energy through braking or deceleration into electric energy which is stored in its battery, ready for re-use. Braking energy is harnessed only through the front wheels, not rear. The Prius also works as a pure electric vehicle (EV) at slow speeds, not using the petrol engine at all.
The modes the Prius can run in are:
- EV (Electric Vehicle). Propelled only by the electric motor. Works only up till around 40km/h, and will take the car up to 2km. On the flat.
- Petrol – the petrol engine only powers the car. The petrol engine may also run a generator to recharge the battery at the same time.
- Combined – the car is driven by both the petrol and electric engines at the same time.
Toyota call this Hybrid Synergy Drive. In practice, the Prius will prefer its electric engine at slow speeds, the petrol at higher, it’ll take every chance it can to recharge its battery, and will use both electric and petrol engines when it needs to accelerate.
The Prius transmission is a CVT, which we have fully explained here. Briefly, a CVT is a ‘gearless’ transmission which allows the engine to remain precisely in its most effective rev range. That’s why Toyota don’t quote how many ‘gears’ the Prius has.
About the i30…
The i30 has the latest turbo-diesel technology. I could fill a book writing about it, but take it as read if there’s something new and cool, the i30’s engine has it. Briefly though – the diesel engine has a variable geometry turbocharger which means the turbo changes configuration so it works efficiently across all rev ranges, and it is common rail which means microscopic droplets of diesel are very precisely metered into the engine under extremely high pressure, all computer controlled, again for maximum efficiency.
The i30 also has a DSG gearbox, which means Direct Shift Gearbox. This is a very efficient automatic transmission that doesn’t have the friction losses of a normal automatic gearbox (one with a ‘torque convertor’), shifts very quickly and has seven gears so the engine can spend a lot of time in its most effective rev range, not too dissimilar to the CVT in that respect, but where the CVT has an infinite amount of gears the DSG has seven.
The one thing the i30 is missing is stop/start tech which turns the engine off when the car comes to a halt, and instantly restarts it.
Diesel vs petrol
Diesel engines are more efficient than petrol engines – you go further on a litre of diesel than petrol. This is because there is more energy in a litre of diesel than a litre of petrol. However, diesel engines are heavier not only through their inherent construction, but because they have turbochargers and intercoolers.
The test route started with Part A: 50km of busy Saturday morning suburban traffic; then Part B, another 90km of faster suburban roads and freeways, again in fairly busy conditions. The total distance was 140km, the maximum speed was 100km/h, average speed was 54km/h and the total time was 2 hours, 35 minutes. Of that, 25 minutes was spent stationary.
Both vehicles were unloaded, with tyre pressures set at manufacturer’s recommended. We brim-filled both tanks at the start of the test and at the end because you cannot rely on the consistency of the automatic click-stop even at the same servo on the same pump. Aircon was on in both cars, and ambient temperate was between 27 and 30 degrees. Both cars have Eco modes, and both are a total waste of time for reasons I will explain later in a later post, so neither mode was enabled. Both vehicles were warmed up before the test commenced.
I drove the i30 in the lead, and used basic fuel efficency driving techniques, but not to the extent we cruised below speed limits, other road users were impaired, or road safety compromised. The Prius followed right behind and did the same.
After Part A, the urban run, the Prius’ internal fuel consumption meter registered 3.6L/100km and the i30 4.5L/100km. While neither internal meter is accurate, they can be used for relative comparison.
The Prius won because:
- It switched its engine off at idle (had the i30 been fitted with idle stop/start tech this advantage would have been largely negated);
- Is 260kg lighter (important for acceleration); and
- Is able to store energy used to brake the car to accelerate and move later on.
The official urban cycle fuel consumption figures are 5.9L and 3.7L for the i30 and Prius respectively. The real-world test shows a similar gap, and both cars beat their official figures with little effort from the drivers – more on why people should stop whining about the official figures is here.
Then we moved to Part B, which involved 80km/h roads (thanks Melbourne, we seem to have quite a few such freeways), less suburban driving and freeway cruising. This is where the tables were turned, as the i30’s fuel consumption crept down. Conversely, the Prius’ edged up. The reasons are more or less the inverse of Part A:
- The Prius had little chance to use its energy recovery systems and idle stop; and
- The weight difference is much less important in cruise than acceleration.
At the end of the rest we had the following results:
|Final results||Distance & time||Indicated fuel use (L/100km)||Fuel used (L)||Actual fuel use (L/100km)||Official combined cycle figure||Total emissions (1)|
|Hyundai i30 diesel auto||140km / 2hr 35m||4.2||5.52||3.9||4.9||18.06kg|
|Toyota Prius C hybrid||140km / 2hr 35m||3.7||5.29||3.8||3.9||12.06kg|
Here’s what we learned based on this test. Obviously different conditions will produce different results, but nevertheless the conclusions here are pretty sound for the average driver.
- Hybrids do not make financial sense. You pay a lot more to buy a hybrid and the fuel consumption savings are minimal at best. The only time a hybrid makes sense is when you do a lot of stop/start driving below an average speed of 50km/h, and even then you’d need to do a lot of driving, an infeasible amount (see below), to get back the thousands of extra dollars you pay for a hybrid over the same-sized conventional car. Once you’re rolling, a modern diesel is more efficient – we need to allow for the fact the i30 is a bigger car than the Prius, and yet it very nearly matched it for fuel efficiency. I feel sure that had the i30 been the same size as the Prius it would have used less fuel;
- Hybrids pollute less – the total emissions are pretty clear, 18 to 12. Even accounting for the i30’s extra size, that’s a win for the Prius; and
- The combined cycle is achievable Yes, the lab-tested fuel consumption figures are not out of reach with some basic fuel consumption techniques.
Hybrids are what you buy if you are willing to pay extra to reduce emissions, or if you really want a electric vehicle mode which would be useful when maneuvering for extended periods of time in confined spaces. Otherwise, a small, modern diesel (or petrol) makes more sense.
(1) how come the cars used 5L or so of fuel (which weighs less than 5kg), yet managed to produce much more weight in CO2 emissions? The answer is that during combustion each carbon atom combines with two oxygen atoms to make CO2, and the emissions figures take this into account. The extra two atoms make the total CO2 result around 3.7 times heavier than the pure carbon alone. What’s important is not so much the actual weight (although less is better), but the relative amount between vehicles.
Hyundai and diesel emissions
In the wake of #dieselgate, Practical Motoring asked Hyundai about ‘defeat devices’and they told us this: “We confirm that none of our vehicles are fitted with any type of software intended to manipulate regulatory emissions test results.”
The question doesn’t apply to the Prius as it’s a petrol.
How far do you need to drive to make money on a more fuel efficient car?
A long way. Taking the i30 and Prius C at $23,990 and $26,391 that’s a $2,401 price saving. If we use the standard fuel consumption of 4.9 and 3.9L/100km then for 20,000km per year we have $1421 and $1076 for fuel, assuming diesel is a bit more expensive – that’s $400 per year saved.
Then divide $2401 by $400 and the answer is about seven years before the extra purchase price is worth it.
That doesn’t account for higher repayments, more money tied up in a car and lots of other factors. If you did the same calculations for diesel vs petrol you find the same thing, the extra cost of a diesel isn’t worth it.
One scenario in which hybrids make sense is as a taxi:
The reason is that taxis spend a long time in stop/start traffic, and at slow speeds. The huge mileages they cover means the hybrid premium is worth paying.
A note about the Hummer and the prius…
Please don’t comment about that study which shows a Hummer has a lower lifecycle cost than a Prius. It is clickbait junk research of the lowest order that has been comprehensively torn apart in many different places so I’m not going to bother with it here. It is also an object lesson in how people uncritically repost information that fits their preconcieved ideas.
However, like most junk research that study does have a fundamentally true premise which is that you have to look at the end-to-end environmental impact of your vehicle, not just the emissions/consumption. In that regard hybrids come off worse than conventional vehicles because they have all the normal parts but an extra engine and battery. How much worse? Depends on the vehicle, and whether or not it’s worse overall also depend on your usage in real life.