Jump to content
Mechanical Engineering

Why Automatic Transmissions are more thirsty for fuel than their Manual counterparts?

Recommended Posts

  • 4 weeks later...

Why Automatic Transmissions are more thirsty for fuel than their Manual


What exactly do you mean by automatic? Anything without a clutch? CVT? DCT (dual clutch)? Traditional?

On older cars the option was often between a manual and an automatic with less gears. So the automatics had some weight penalty, a slight friction penalty (related to the torque converter), and less gears. As a result the engine wouldn't run as close to the optimal RPM resulting in worse MPG.

However this trend has reversed and often the automatic has an infinite number of gears (CVT), more gears (up to 9), or at least as many. As a result often the automatic has a higher MPG than a manual. Additionally in some cases the automatic also results in better performance, although that is still less common. In particular the DCT (dual clutch) option on high end sports cars often get to 60 quicker than the manual version

Link to post
Share on other sites
  • 1 year later...
  • 3 months later...

Automatic transmissions (when new) are extremely fuel efficient.Gear ratios and engine speed/load are superbly matched especially with the use of modern  electronics-speed sensors and transmission control modules.The risk of driving in wrong gear which burns more fuel does not exist as gear up-shift or down-shift is electronically controlled.Even in the earlier all hydraulic automatics the merit of  better fuel economy than manuals was there. 

I do not understand when it is suggested here that automatics are fuel thirsty.

Link to post
Share on other sites
  • 1 month later...

The main reason was the torque converter. Until recently, the torque converter only transmitted maybe 95-98% of the engine's power to the actual gear cluster. Then, there is the parasitic loss within the planetary gear set before final drive. And the additional requirement to drive the fluid pump that provided hydraulic pressure to operate the valve body and clutches. Manual transmissions only have parasitic loss in the gear set. The clutch transmits 100% of the power and there is no fluid pump. Today, there are claims that a modern automatic is more efficient than manual, with multiple gears and a logic-controlled shift pattern.

Link to post
Share on other sites
  • 2 years later...

Automatic transmission cars get less fuel efficiency because:

1. They weigh more.

2. The torque converter is a fluid coupling that allows some slippage while the engine slows to the lower RPM on an upshift or a higher RPM on a downshift.  That slippage is inefficient (turns power to heat).

Modern automatics have a torque converter lockup that locks the slippage once the gear is shifted and the engine settled to its new speed, so the loss is less severe than older models where there was always slippage.

Many automatics don't engage the torque converter lockup in low gears or when the transmission is not warmed up.

Link to post
Share on other sites

Join the conversation

You can post now and register later. If you have an account, sign in now to post with your account.

Reply to this topic...

×   Pasted as rich text.   Paste as plain text instead

  Only 75 emoji are allowed.

×   Your link has been automatically embedded.   Display as a link instead

×   Your previous content has been restored.   Clear editor

×   You cannot paste images directly. Upload or insert images from URL.

  • Join Mechanical Engineering network

    Join us (login) to get full access : Please sign up to connect and participate.

    To download files...please login

  • Create New...