Learn the facts: Variable valve timing and its impact on fuel consumption

What is the issue?

Progressively more stringent greenhouse gas emission standards for light-duty vehicles are in place in Canada, leading vehicle manufacturers to improve engine efficiency by using innovative technologies such as variable valve timing (VVT).

What do I need to know?

VVT and lift systems optimize the intake and exhaust valve timing relative to the engine speed, in contrast to conventional systems in which the valve timing and lift are fixed. This variability allows for improved engine efficiency and power over a broad range of engine operating speeds. VVT and lift can reduce fuel consumption by 1 to 6%, saving you money and reducing your impact on the environment.

How does variable valve timing work?

  • Just like humans, a car engine needs to breathe. Engine valves control the intake of fresh air and the fuel mixture and the exhaust of combustion gases.  And, just like humans, an engine’s breathing rate is based on exertion; VVT adjusts the engine’s timing and breathing based on the operating conditions of the engine. Conventional valve control systems provide a single fixed timing duration, which means engine breathing is not adjusted or optimized for the differing requirements.
  • Good timing between the intake and exhaust valves is crucial for optimal engine efficiency. If this timing can be controlled and changed in real-time, based on engine operating parameters, the engine could perform better. The engine would need less fuel to provide the same amount of power.
  • VVT helps to optimize combustion, which provides several benefits, including improved engine operation, increased engine power (which allows the use of a smaller engine to achieve the same performance), reduced fuel consumption and decreased emissions.
  • Manufacturers have adopted various VVT design approaches and technologies to control the timing and how long the intake and exhaust valves remain open. The simpler approaches provide just two timing modes (e.g. a camshaft that has lobes of different heights used for lower and higher engine speeds). More sophisticated approaches provide continuously variable timing (e.g. a camshaft that dynamically adjusts its position and timing).
Intake stroke for an engine equipped with VVT and an engine without; note the timing is different

Figure 1: Intake stroke for an engine equipped with VVT and an engine without; note the timing is different. Illustration taken from Toyota’s VVT-I video available on YouTube.

How can I help?

Be a knowledgeable buyer. Research before you buy and include a lifetime estimate of fuel consumption as a cost and performance requirement.

What are the savings and benefits?

Improvements in engine technology and efficiency can save you money, as illustrated in the following table. VVT can reduce fuel consumption and emissions by 1 to 6% compared to conventional technology. Over 10 years, this reduction corresponds to fuel cost savings of $134 to $1,828 and carbon dioxide (CO2) reductions of 280 to 3,860 kg. At the high end, this is equivalent to:

  • three quarters of an Olympic-sized swimming pool of CO2
  • removing a mid-size car from Canadian roads for one year
  • nearly 20% of our annual per capita emissions in Canada, which is 22.1 tonnes
The table illustrates how improvements in engine technology and efficiency can save you money.
Fuel consumption Potential annual savings Potential 10-yr savings
Average
(L/100 km)
With a 1% reduction
(L/100 km)
With a 6% reduction
(L/100 km)
Fuel cost savings CO2 reduction Fuel cost savings CO2 reduction
14.0 13.86 13.16  $31-183 64-386 kg $305-1,831 640-3,860 kg
12.0 11.88 11.28  $26-157 55-331 kg $262-1,570 550-3,310 kg
10.0 9.90 9.40  $22-131 46-276 kg $218-1,308 460-2,760 kg
8.0 7.92 7.52  $17-105 37-221 kg $174-1,046 370-2,210 kg
6.0 5.94 5.64  $13-78 28-166 kg $131-785 280-1,660 kg

Note: For illustrative purposes, savings are based on an annual driving distance of 20,000 km, a fuel price of $1.09/L and a CO2 emissions factor of 2.3 kg/L of gasoline.