Corona goes the distance

100mpg to be precise. Toyota has developed a super-efficient engine that could revolutionise the automotive industry, reports Paul Carslake

From the outside, it looks like an ordinary, reasonably spacious family car. But under the bonnet of this Toyota Corona is a drive system that marks a world first in automotive engineering.

This steel-bodied car can travel 100 miles on a single gallon of petrol – with acceleration and performance on a par with any conventional rival, and half the carbon dioxide emissions. The secret is a super-efficient petrol engine linked with a powerful electric motor, what the industry calls a hybrid power train. The Toyota Hybrid System looks set to be the first such powertrain to go on sale in a mainstream passenger car.

The principle behind the THS is simple. A hybrid uses a combination of two power sources. Each power source maximises its advantages, while offsetting the disadvantages of the other.

In the THS, electricity generated by the petrol engine is used either to drive the electric motor or is stored in a battery. The system chooses to power the vehicle with motor or engine, depending on the speed and driving conditions.

Concepts for hybrid vehicles have been doing the rounds at motor shows for the past decade, and have developed mainly because of the shortcomings of battery technology. If batteries could hold more power for longer and recharge more rapidly, electric cars would be a hit. But battery-powered cars – including General Motors’ pricey EV1 on sale in parts of the US – have limited range and performance. Not surprisingly, plenty of car makers have been looking at hybrid powertrains to give clean and quiet urban driving with the ability to make long-haul motorway trips too.

Car makers have a choice of two types of hybrid system – series or parallel. The series hybrid uses an engine to drive a generator which in turn powers the electric motor that drives the wheels. In a parallel system, the power is split so that both engine and motor drive the vehicle. The Toyota system is a smart version of the parallel hybrid: at times, the engine is shut down altogether, and the engine/motor power split is continuously variable.

But while rival manufacturers’ concepts are still out on the proving grounds later this year, Toyota’s hybrid-powered car will be in Japanese showrooms. The price will be heavily subsidised, allowing buyers to pay only about 50% more for THS power than they would for an ordinary petrol engined family saloon.

The hybrid Corona has proved that the system can work. The cars that go on sale this winter though, will look very different. They will be unveiled at the Tokyo Auto Show in October, billed as the cars for the 21st century. They will look futuristic, though details remain top secret. ‘But it will be a highly advanced vehicle worthy of the hybrid power system,’ says a Toyota source.

Whatever the bodywork will look like, the heart of the THS remains a set of cogs forming a planetary gear that effects the power split. This allows a continuously variable mix of engine power to be transmitted to the generator and to the driveshaft.

The generator provides electricity to drive the motor or recharge the battery. The amount of electricity generated can be controlled electronically by governing the speed of revolution of the generator. This effectively determines the ratio of the power split. The faster the generator spins, the less engine power goes directly through to the driveshaft.

Linking a petrol engine with a generator and motor in this way unlocks the potential to build a more efficient engine, as there is no need to meet conventional requirements such as good throttle response or a wide range of operating speeds.

The 1.5 litre, four cylinder, 16 valve petrol engine is all-new and operates at maximum thermal efficiency based on the efficient operating strategy of the THS rather than at maximum output – as would be the case with a conventional engine.

Toyota engineers have also been able to build in Atkinson cycle valve timing – which converts more of the combustion energy into crankshaft revolutions. The engine achieves optimal fuel consumption when operating in its highest torque ranges and is electronically controlled to stay within this range, with a maximum speed of just 4,000rpm. This limitation creates extra benefits, allowing some of the engine components – such as the crankshaft – to be smaller and lighter.

All this saves fuel, not least because the engine never has to operate in less efficient modes. At low speeds, the engine is shut down and the electric motor drives the car. As the vehicle’s speed increases, the engine is started up (cranked by the generator) and moves directly to its most efficient running speed.

Throttle control is electronic, so depressing the accelerator simply sends a request to the processing unit to make the vehicle increase speed. Electronics then decide which power option to bring into play, depending on the speed the vehicle is travelling at the time. At low vehicle speeds, this overall force is dominated by the motor.

This arrangement means there is no need for a separate transmission – a vital space-saving element of the design. The complete THS transmission – with planetary gear power split device, generator and motor – acts like an electronically-controlled continuously variable transmission. And it fits into a space roughly equivalent to a conventional five-speed gearbox, allowing the hybrid system to be installed within the architecture of a conventional mass-produced vehicle.

The initial production levels look set to be small at about 1,000 units per month, with mass production planned for perhaps a year later. A global launch has been delayed until after the engine has proved itself in Japan. Export sales to Europe and the US could start in 2001.

Consumer reaction to the initial run of THS cars in Japan will be crucial, although Toyota’s biggest challenge is now commercial: how to get to the point where it can sell cars like this at a profit.

How the Toyota Hybrid System works

Power from the engine is split to power the wheels directly and the electric motor (via a generator). Engine or motor or both power the car depending on the driving conditions.

Starting out/light load

When starting out, crawling through city traffic or going down a moderate slope, the petrol engine shuts down as it would not be operating at peak efficiency. The electric motor (A) takes over.

Normal driving

The engine’s power is split down two paths. One (B) drives the wheels and the other (C) drives a generator to produce electricity for the motor, which in turn adds driving force to the wheels. The share of how much power goes down which path is decided electronically, with the aim of keeping the engine running at its most efficient rev range.

Full throttle acceleration

To maximise power available to the driving wheels, electricity is drawn from the battery (A) to boost the performance of the electric motor. The petrol engine (B) is also driving the wheels.


Hit the brake pedal and the hydraulic brakes are activated in the normal way, but the inertia of the wheels is used to turn the motor, which acts as a generator to store electricity in the nickel-metal hydride battery (A) and adds additional braking force – a process known as regenerative braking.

Battery recharging

The battery is monitored to maintain a constant charge. When it gets low – after extended travel at full throttle, for example – the generator routes power from the engine (B) to recharge it.

{{THS at a glance

Engine: 1.5 litre, 4 cylinder fuel-injected petrol, DOHC 16-valve, intelligent variable valve timing.Electric motorand generator: Permanent magnet synchronousBattery: Nickel-metal hydridePower splitdevice: Planetary gear}}