On-road engine stop/start and parked-cab cooling need separate business cases. One changes how a truck uses fuel while travelling; the other addresses comfort during stationary periods. A saving reported for one system should not be presented as evidence for the other.

What Volvo announced

Volvo Trucks' 4 November 2025 announcement describes an I-Roll engine stop/start function that temporarily stops the engine in suitable driving conditions. The release says activation is above 60 km/h and reports potential additional savings subject to operating conditions. It is not an overnight parking-cooling test.

Divide the vehicle's day into operating states

For a fleet study, record time spent driving, loading, waiting, resting and parked without an occupant. Add the reason the engine runs during stationary periods. Some engine operation may support a required vehicle function, while other operation may be primarily for cabin comfort. Treating all stationary engine hours as avoidable can overstate the opportunity.

Then identify the subset of those hours a proposed cooling system can actually replace. Match that period with the system's usable energy, cooling capability and operating limits. If the battery runs out before the rest period ends, count the remaining time separately.

Use a transparent calculation

A planning model can use avoided cooling-related engine hours multiplied by measured fuel consumption during those hours. Subtract the cost of replenishing energy and the incremental maintenance costs of the alternative. Keep equipment purchase and installation as separate investment inputs.

The model should show where each input came from. A telematics record, a measured idle test and an assumed fuel price carry different levels of confidence. Include a range for uncertain inputs so that the decision does not depend on one optimistic figure.

Check the driver's experience

Reduced engine hours are useful only if the proposed arrangement meets the required cabin conditions. Record the temperature at the bunk, interruptions and the driver's ability to use the controls during a trial. Avoid treating a switched-off engine as proof of a successful comfort outcome.

Fleet buyers can consider driving-efficiency improvements and parking cooling as complementary projects. Give each its own baseline, test and results. That approach helps finance and operations understand what changed and prevents a vehicle manufacturer's road-test claim from becoming an unsupported promise about an aftermarket air conditioner.

Log both systems in one data sheet

A fleet that wants honest numbers puts I-Roll events and parking-cooling events into the same sheet, with one row per operating day. Each row answers the same questions: how far the truck drove, how long the engine ran while stationary, what the stationary engine time was for, and how much energy an added cooling system drew. When both topics live in one structure, the comparison can be made route by route instead of as two unrelated averages.

Keep the columns separate even though the rows are shared. A stop/start event on the road changes fuel use during travel; a parking air conditioner changes fuel use during rest. Merging them into a single "savings" column destroys the distinction this article is arguing for. The sheet below is a starting structure that most telematics exports can populate with little manual work.

  • Date, route or duty number, and the driver identifier.
  • Total driving time and total distance for the day.
  • Stationary engine-running hours, split by stated reason.
  • Occupied rest hours and the location type for each rest.
  • Energy delivered to the cooling system, from its own metering if available.
  • Fuel filled, so daily consumption can be reconstructed at the end of the period.
  • Ambient conditions, at least as a coarse hot, mild or cold classification.
  • Notes on anything unusual: detours, a failed load, a different trailer.
  • Which system was active for parked cooling that night, so the two weeks stay comparable.

Assign the sheet an owner before the first row is written. In most fleets the natural owner is a dispatcher or maintenance planner who already touches the daily records, not a driver and not an analyst. The owner fills gaps, chases missing fuel entries and signs each weekly review. Studies without a named owner produce beautiful first weeks and holes thereafter, because recording is everyone's job and therefore nobody's. Put the owner's name in the sheet header itself, so the responsibility travels with the file when it is emailed or printed for a review meeting.

Review the sheet weekly rather than at the end of the trial only. Weekly review catches empty columns, a driver who interprets "reason" differently from the rest of the fleet, and meters that stopped recording. Fixing those during the trial costs a phone call; discovering them afterwards costs the whole measurement period.

Which stationary hours can a parking AC actually replace?

The honest answer is: fewer than most first estimates assume. A parking air conditioner can replace engine hours that run purely for cabin comfort during an occupied stationary period, within the energy the vehicle can supply and the unit can store. It cannot replace engine hours that support a required vehicle function, and it cannot cover a rest period longer than its energy budget. Sorting stationary hours into these groups is the core of the whole study.

  1. List every stationary engine-running situation on the route, however brief.
  2. Mark whether a driver is present and whether the cab must stay conditioned.
  3. Mark whether the engine run supports a required function rather than comfort.
  4. Estimate the duration a cooling unit would need for each marked period.
  5. Compare that duration with the energy available from the electrical system.
  6. Keep the remainder as engine hours that no cooling system will remove.
  7. Re-run the grouping whenever the roster or route structure changes.

Two boundary cases deserve explicit decisions. A pre-cooling period before the driver boards may look avoidable but serves a load or animal-care requirement on some operations; decide per route, not in general. Overnight idling in jurisdictions where local rules restrict engine running is not a free choice either — the comparison there is between a cooling system and a compliance problem, which changes its value entirely.

Shift pattern shapes the answer more than truck specification does. A driver on a fixed schedule with guaranteed depot nights has long, predictable rest periods — easy for any system to serve. A driver on irregular loading, split shifts or hot courier work has short occupied stops the cooling unit can reach only partly, separated by unoccupied gaps nobody needs to condition. Model the second pattern honestly and the replaceable share shrinks again. This is why the grouping exercise must use the fleet's own rosters, not an industry-average night, and why the result should be re-checked whenever the roster logic changes.

When the grouping is done, the replaceable share is usually a minority of stationary engine hours. That is not a failure of the technology; it is what a correct boundary looks like. A business case built on the replaceable share survives scrutiny, while one built on all stationary hours collapses the first time an operations manager reads the assumptions.

Mistakes that flatter a stop/start comparison

Errors in these studies are rarely deliberate; they creep in through convenience. The dangerous ones all move savings in the same direction — upward — which is why they survive review. Check a plan against the list below before any figure is presented to finance, because every item on it has ended real fleet projects.

  • Attributing I-Roll road savings to parking comfort. The announced function operates above 60 km/h in suitable driving conditions; it does nothing while the truck is parked, so its savings belong in the driving column only.
  • Using a single measured idle test for every season. Fuel use at idle rises with cooling demand; a spring measurement understates summer and overstates winter. Repeat the test in the seasons that matter to the route.
  • Crediting both projects for the same engine hour. If a rest-stop hour is counted as avoided by the cooling system, it must not also appear as an efficiency win elsewhere in the model.
  • Taking cooling capacity from brochures instead of trials. Rated figures describe laboratory conditions; the trial record from the previous section is the number that belongs in the model.
  • Ignoring the cost of replenishing energy. Charging takes fuel or shore power or driving time; it is small per day but not zero, and pretending otherwise overstates the net saving.
  • Presenting a single-truck result as a fleet average. One truck proves the mechanism, not the size; scale only comes from the wave structure described later in this article.

One structurally different error is sampling only well-behaved routes. A trial on the smoothest long-haul lane says little about a regional duty with short hauls and long waits. If the fleet intends to roll out widely, at least one trial route should be chosen for being awkward, not for being easy to measure.

If a model already exists, audit it by recomputation rather than by discussion. Take the sheet, recompute the saving from the raw rows and compare the result with the presented figure; where the two diverge, the difference will sit in one of the mistakes listed above. An audit that only reads the assumptions tends to confirm them, because every assumption looks reasonable in isolation. The recomputation takes an afternoon, requires no permission from whoever built the model, and either certifies the number or finds the exact row where optimism entered the calculation.

A two-week trial on one truck

A single truck, a single driver and two weeks produce enough evidence for a go or no-go on a wider program. Week one runs the existing arrangement — I-Roll active, engine used for parked cooling as today — and establishes the baseline. Week two adds the parking air conditioner and repeats the same routes and rest pattern as closely as operations allow. Everything measured in week one is measured again in week two with the same instruments.

  1. Fix the route, the trailer type and the rest locations for both weeks.
  2. Install meters before day one; never add instrumentation mid-trial.
  3. Record week one completely, including days the driver considers unremarkable.
  4. Install and commission the cooling unit between the weeks, with records kept.
  5. Run week two on the same pattern and log energy and temperatures daily.
  6. Close with a joint review: driver, dispatcher and whoever will fund the next step.
  7. Archive both weeks raw; summaries can be rebuilt, raw rows cannot.

Choose instruments once and leave them alone. A fuel-flow meter and a simple energy meter on the cooling unit cover the essential numbers; cab temperature can be logged by an inexpensive sensor placed at the bunk, away from the outlet airflow. What must not happen is a mid-trial upgrade to better instruments, because it breaks the pairing between the weeks. If the first meters prove unreliable, note which days are affected and extend the trial rather than switching devices in the middle — a slightly longer trial with consistent tools beats a short one with a gap in the record.

Guard the trial's integrity at the boundaries. The driver should not know interim results, because a motivated driver can unconsciously extend rest periods to help the new system. Operations should resist reassigning the truck mid-week; one swapped route is a footnote, three are a different study. And every anomaly — a breakdown, a weather event, a changed loading — goes into the notes column, so the final review can decide its weight instead of arguing about memory.

The deliverable is not a verdict but a file: two weeks of paired data, the commissioning record, the driver's comments and the meter photographs. That file is what turns a promising trial into a procurement decision, because it lets a second person recompute the result independently.

Questions for the dealer and the cooling supplier

Two suppliers sit on either side of this comparison, and each should be asked about its own system only. The truck dealer answers for I-Roll behavior and the vehicle's electrical approvals; the air-conditioning supplier answers for energy use, installation and service. Mixing the questions produces mixed answers, so split the meeting or send two separate requests.

  • To the truck dealer: which operating conditions permit the stop/start function on our duty cycle, and where is that documented?
  • To the truck dealer: what electrical load limits apply to accessories while the vehicle is parked, and who approved them?
  • To the cooling supplier: what energy does the proposed unit draw over a full rest period at our measured cab temperatures?
  • To the cooling supplier: what installation, wiring protection and commissioning does this specific vehicle need, and what service terms apply afterwards?

Ask both parties the same final question: what would make this project fail? A supplier who can name the failure conditions — a rest period longer than the energy budget, a route with no charging window, an electrical limit lower than assumed — is a supplier who has done the engineering. An answer of "it will all work out" is itself a finding, and it belongs in the comparison file next to the price.

Insist on answers in writing, tied to the route data the fleet supplied. Spoken answers drift; a document can be quoted in a purchase order. When a supplier's written answer conflicts with the trial record — claimed energy draw below the metered draw, for instance — resolve it before contracting, not after commissioning. The comparison file from the two-week trial gives both suppliers the same operating problem, which is what makes their answers comparable at all. Without shared data, each quotation prices a different invisible truck, and the cheaper offer may simply be the less honest one.

Acceptance criteria before a fleet decision

Before the pilot expands, agree in writing on what the wider rollout must prove. The criteria below are deliberately measurable; each one can be checked from the trial file without interpretation. A project that meets them has earned its scale-up, and one that fails a criterion has a documented reason to pause rather than a reputation to defend.

  • Net saving per truck per month, computed from paired weeks, exceeds the agreed threshold.
  • The replaceable share of stationary hours, from the grouping exercise, matches the model's assumption.
  • Cab temperature during occupied rest meets the standard the drivers' representative signed.
  • The electrical installation passed inspection, and the charging routine survived a full duty cycle.
  • Drivers report, in the structured form, no loss of rest quality or control convenience.
  • Service terms — response time, spare parts, warranty path — are documented for the cooling unit.
  • The maintenance schedule names who inspects the electrical installation and how often.

Scale in waves rather than all at once. Ten trucks in the second wave reveal installation differences that one pilot truck conceals: two body configurations, two drivers' habits, two depots with different shore power. Wave two should repeat the trial structure in miniature, with the same sheet and the same review cadence, and its results should be compared with wave one before wave three is funded. This costs a few weeks and prevents a program that looks successful in aggregate while hiding a group of trucks where the numbers never worked.

Keep the two projects' identities separate in every document that leaves the fleet, including internal ones. Reports, purchase orders and meeting minutes should name the on-road stop/start project and the parked-cooling project by different titles, with different owners and different baselines. The vocabulary sounds like bureaucracy until a problem appears; then separate identities decide whether the right project is paused or the wrong one. The distinction this article opened with — travel savings versus parked comfort — is preserved or lost in exactly these routine papers.

Finally, decide the reporting rhythm before scaling. The first quarter after rollout should reproduce the two-week data sheet monthly, so early drift — a broken meter, a bypassed routine, a route change — appears while it is still cheap to fix. Efficiency projects decay silently when nobody measures them; the sheet that proved the concept is the same sheet that keeps it honest.

Next step

Compare the available configurations in the Vethy product catalog, then request the current model-specific documentation before selecting a unit.

Sources and method

Frequently asked questions

Does Volvo I-Roll reduce fuel use during overnight parking?

No. The announcement describes a stop/start function that temporarily stops the engine in suitable driving conditions, with activation above 60 km/h. Overnight parking is a different operating state: the truck is stationary, and any engine running there serves rest comfort or another stationary need. Savings reported for on-road stop/start must not be presented as parking savings. Fleets should measure the two separately, using the operating-state log and the paired-week trial structure described in this article, before either figure enters a business case.

Can a parking air conditioner replace engine idling at night?

It can replace the portion of idling that runs purely for cabin comfort, within the limits of the available energy and the unit capability. The correct method is to group stationary engine hours by purpose, mark the comfort-only share, and match that share against the energy the electrical system can deliver and the battery can store for the rest period. Whatever remains — engine hours tied to required functions or beyond the energy budget — stays as engine time. A claim built on that boundary holds up when operations and finance review it.

How should a fleet calculate the saving from a parking air conditioner?

Use avoided cooling-related engine hours multiplied by measured fuel consumption during those hours, then subtract the cost of replenishing the cooling system energy and the incremental maintenance of the added equipment. Keep purchase and installation as separate investment inputs, not as a smear across monthly savings. State where each input came from — telematics record, measured idle test, assumed price — and carry a range for the uncertain ones. A transparent calculation with a stated low case survives review better than a single optimistic figure.

Are I-Roll savings and parking AC savings additive?

They can be, because the two act on different operating states: one reduces fuel during travel above 60 km/h, the other replaces engine running during stationary periods. But additivity must be demonstrated, not assumed. The same truck must be measured with a clear operating-state log that separates driving hours from stationary hours, and no engine hour may be counted in both columns. Where the fleet runs mixed duty cycles, run the demonstration per duty cycle; a sum that holds on long-haul lanes can fail on regional routes with long stationary periods.